Preheat your oven to 375°F. Let it fully preheat, don't rush this. An oven that hasn't reached temperature means the topping sets before it can brown properly.
Slice chicken breasts horizontally into ½-inch cutlets if they're thick. Uneven pieces mean some dry out while others stay undercooked. Even thickness is everything in a casserole.
Season the chicken lightly with salt, pepper, and a pinch of garlic powder on both sides. Lay them flat in a single layer in a lightly greased baking dish, no overlapping.
Mix Greek yogurt, grated parmesan, and garlic powder in a bowl until fully combined. Taste it, it should be bold and savory. Add a pinch of salt if needed.
Spread the yogurt-parmesan mixture generously over every piece of chicken. Cover every edge. Pile it thick, this is not the place to be conservative.
Bake uncovered at 375°F for 30–35 minutes until the topping is deeply golden, bubbling at the edges, and the thickest piece of chicken reads 165°F on a meat thermometer.
Broil for the final 2–3 minutes if you want an extra-deep, almost charred crust on top. Watch it closely, it goes from golden to burnt in under 60 seconds under the broiler.
Rest for 5 minutes before serving. The yogurt topping continues to set as it cools slightly, cutting into it immediately makes it slide off. Five minutes is all it needs.