Preheat and prep: Heat the oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish or a similar casserole dish.
Season the chicken: Pat the chicken dry.
In a bowl, toss cubes with salt, pepper, garlic powder, onion powder, Italian seasoning, and olive oil until evenly coated.
Make the sauce mix: In a separate bowl, stir together marinara, Greek yogurt, 1/2 cup mozzarella, 1/4 cup Parmesan, and red pepper flakes if using. Taste and adjust salt if needed.
Assemble the base: Spread half the sauce mixture across the bottom of the dish. Scatter the seasoned chicken in an even layer on top.
Add the rest of the sauce: Spoon the remaining sauce mixture over the chicken.
Don’t worry about perfect coverage—just aim for most pieces to be saucy.
Top with crunch: In a small bowl, mix panko with melted butter until evenly moistened. Sprinkle the crumbs over the casserole. Finish with the remaining 1/4 cup mozzarella on top.
Bake: Place on the middle rack and bake for 22–28 minutes, until the chicken reaches an internal temperature of 165°F (74°C) and the top is golden and bubbly.
If the crumbs need more color, broil for 1–2 minutes, watching closely.
Rest and garnish: Let the bake rest for 5 minutes to settle. Shower with extra Parmesan and chopped basil or parsley.
Serve: Pair with a simple green salad, steamed broccoli, spaghetti squash, or a smaller portion of whole-grain pasta.