Last weekend, as the first real frost painted my neighbor’s roof white, I found myself craving something that would wrap the kitchen in warmth and keep my post-workout macros on track. I’d just lugged in a crateful of butternut squash, Brussels sprouts, and the last local spinach of the season, plus two pounds of chicken breast that was on manager’s special. One glance and I knew: a high-protein chicken and spinach casserole with roasted winter vegetables was destiny. Fifty minutes later—after the oven had done its magic—my husband, the self-proclaimed casserole skeptic, took a bite and quietly carried the entire 9-by-13 dish to the table. No leftovers for the freezer that night; only smiles and requests for “that one” to headline every cold-weather Sunday supper from here on out. Whether you’re feeding teenagers after hockey practice, looking for make-ahead lunches that won’t leave you hungry by 3 p.m., or simply wanting a one-dish wonder that tastes like winter comfort but fuels like gym food, this recipe is for you.

Why This Recipe Works

High-protein powerhouse: Each generous serving delivers 42 g of complete protein—enough to satisfy even the pickiest macro counters.
One-pan convenience: Roast the vegetables while you whisk the sauce; everything finishes together for minimal cleanup.
Seasonal flexibility: Swap in any sturdy winter veg—think cauliflower, sweet potato, or even beets—without touching the macros.
No canned soup shortcuts: A light, cheesy Greek-yogurt base keeps sodium in check and flavor bright.
Freezer-friendly: Assemble on Sunday, freeze raw, then bake straight from frozen on a busy Wednesday.
Kid-approved texture: Finely chopped spinach melts into the sauce, so even greens-haters spoon it up.

Ingredients You'll Need

For the best flavor, buy chicken breasts of even thickness so they cook uniformly; if they’re ballooned in the middle, pound them gently between plastic wrap. Look for baby spinach sold loose in bins rather than cello bags—it’s typically younger, more tender, and less gritty. When choosing butternut squash, pick one that feels heavy for its size with matte, unblemished skin; a glossy surface often signals under-ripeness. Brussels sprouts should be tight, tiny, and bright green; yellowing outer leaves mean they’ve been off the stalk too long. Finally, reach for block-style Greek feta stored in brine; pre-crumbled versions are handy but contain anti-caking agents that mute tanginess.

How to Make High Protein Chicken and Spinach Casserole with Roasted Winter Vegetables

Heat the oven and prep your pans

Position two racks: one in the upper third to roast vegetables, one in the center for the final casserole. Preheat to 425 °F (220 °C). Line a rimmed sheet pan with parchment; lightly grease a 9-by-13-inch (3-quart) casserole dish.

Season and sear the chicken

Pat 2 lb (900 g) chicken breast dry; season with 1 tsp kosher salt, ½ tsp black pepper, 1 tsp dried thyme, and ½ tsp smoked paprika. Warm 1 Tbsp olive oil in a skillet over medium-high. Sear chicken 2 min per side—no need to cook through; this locks in flavor. Transfer to a plate to cool slightly, then chop into ¾-inch cubes.

Roast the winter vegetables

Dice 1 small butternut squash (about 1 lb) and halve 12 oz Brussels sprouts. Toss with 1 Tbsp olive oil, ½ tsp salt, and ¼ tsp pepper. Spread on the prepared sheet pan. Roast 18 min, stirring once, until edges caramelize and a paring knife slides in with slight resistance.

Make the protein-packed sauce

While vegetables roast, whisk 1 cup plain non-fat Greek yogurt, ¾ cup low-sodium chicken broth, 2 large eggs, 2 Tbsp Dijon mustard, 1 tsp garlic powder, and ¼ tsp nutmeg. The mixture will look thin; that’s perfect—it thickens as it bakes.

Wilt spinach in seconds

Place 4 packed cups baby spinach in a microwave-safe bowl with 1 tsp water. Cover and microwave 45 sec, just until bright green and collapsed. Squeeze out excess moisture, then roughly chop.

Assemble layers

Scatter roasted vegetables into the greased casserole. Top evenly with chicken cubes, then chopped spinach. Pour yogurt sauce over all; gently shake the dish so sauce filters through. Sprinkle ½ cup crumbled feta and ¼ cup grated Parmesan on top.

Bake to creamy perfection

Cover loosely with foil and bake on the center rack 15 min. Remove foil; continue baking 12–15 min more until the sauce bubbles at the edges and the internal temperature reaches 165 °F (74 °C). Broil 2 min for bronzed cheese if desired.

Rest and serve

Let stand 10 min; this sets the sauce and prevents tongue-scalding. Sprinkle with chopped parsley or pomegranate arils for festive color. Slice into six generous squares and serve hot.

Expert Tips

Cube uniformly

Keep chicken and squash pieces the same size so every forkful cooks evenly—no rubbery nuggets, no crunchy centers.

Patience = fond

Allow the skillet to get ripping hot before adding chicken; that caramelized crust equals deeper casserole flavor.

Sauce thickness test

If you prefer a looser sauce, add 2 extra Tbsp broth; for a strata-style slice, cut liquid by the same amount.

Cheese swap rules

Feta too salty? Trade for ½ cup shredded part-skim mozzarella and add 1 tsp lemon zest for brightness.

Vegetarian option

Sub 2 cans no-salt chickpeas, rinsed, for chicken; roast 10 min with veggies to prevent mushiness.

Spice it up

For a smoky kick, whisk 1 tsp chipotle powder into the yogurt base; you’ll never go back to plain.

Variations to Try

Mediterranean: Swap butternut for zucchini, add sun-dried tomatoes, and use oregano instead of thyme.
Curried harvest: Add 1 Tbsp yellow curry powder to the sauce and substitute sweet potatoes for squash.
Buffalo style: Replace Dijon with ¼ cup Buffalo sauce and stir in ½ cup blue cheese crumbles.
Low-carb: Skip squash, double Brussels, and use ¾ cup liquid egg whites in place of whole eggs.

Storage Tips

Refrigerate: Cool completely, then portion into airtight glass boxes. Refrigerate up to 4 days; reheat single portions 90 sec in the microwave or 15 min in a 350 °F oven covered with foil.

Freeze (before baking): Assemble in a disposable foil pan, wrap twice with plastic then foil, label, and freeze up to 3 months. Bake from frozen at 375 °F for 60–70 min, adding foil if top browns too quickly.

Freeze (after baking): Cut cooled casserole into squares, flash-freeze on a tray, then transfer to zip bags. Warm individual squares 5 min in the microwave or 20 min in the oven.

Frequently Asked Questions

Yes! Stir in 3 cups shredded cooked chicken when you combine vegetables and spinach; reduce final bake time by 5 min since you’re only heating through.

Temper it: whisk hot broth 1 Tbsp at a time into the yogurt until warm, then combine with remaining liquid. Keeping oven temperature under 425 °F also prevents curdling.

Absolutely. Roast vegetables and cube chicken; store separately. Whisk sauce and refrigerate in a jar. Layer everything in the morning, then bake that evening.

Use cauliflower florets or diced carrots instead. Cooking times stay identical; just keep the total veg weight around 1 ¾ lb.

For higher protein you can sub ½ cup blended cottage cheese for part of the yogurt, but keep at least ½ cup yogurt for tang and stability.