If using fresh spinach, steam it in a pot with a little water for 3-4 minutes until wilted, then transfer to a fine mesh strainer and squeeze out as much water as possible—this is crucial for preventing a watery casserole. If using frozen spinach (as the ingredient list suggests), thaw it completely and squeeze it very dry using paper towels or a clean kitchen towel, pressing firmly to remove excess moisture. While the spinach drains, finely dice the half onion and have all remaining ingredients measured and ready.

In a large mixing bowl, combine the Greek yogurt, mayonnaise, garlic powder, lemon juice, and red pepper flakes, stirring until smooth. Add the freshly grated Parmesan and sharp cheddar, then fold in the diced onion until evenly distributed. I like to taste the mixture at this point and adjust seasoning if needed—sometimes I add a small pinch of salt to brighten the flavors. This base should be creamy and well-combined.

Preheat your oven to 350°F. While it heats, lightly grease a 9x13 inch baking dish or similar-sized casserole dish with butter or cooking spray. Spread about half of the creamy cheese mixture from Step 2 evenly across the bottom of the dish, creating a base layer that will help prevent sticking and add flavor throughout.

Distribute the shredded or cubed chicken evenly over the cheese base, breaking up any large clumps so it cooks evenly. Spread the prepared spinach from Step 1 over the chicken layer, then top everything with the remaining half of the cheese mixture from Step 2, spreading it gently to cover and seal in the filling. I find that using an offset spatula helps distribute the topping evenly without compacting the layers underneath.

Place the casserole in the preheated 350°F oven and bake for 40-45 minutes, until the top is golden brown and the filling is bubbling slightly around the edges. The casserole is done when a knife inserted in the center comes out hot and the cheese is melted and beginning to brown on top.

If you'd like a more deeply browned, slightly crispy top, move the casserole to the top oven rack and broil for 1-2 minutes, watching carefully to avoid burning. This step is entirely optional but adds nice textural contrast. Remove from the oven and let rest for 5 minutes before serving.