Preheat Oven: Set oven to 400°F (200°C). Lightly grease a 9x13 baking dish (if not using an oven-safe skillet).
Sear Chicken: Heat 2 tbsp olive oil in a large skillet over medium-high heat. Add seasoned chicken cubes and brown for 3–4 minutes until golden. (Chicken will not be fully cooked).
Sauté Mushrooms: Add 12 oz sliced mushrooms to the skillet. Cook for 5 minutes until moisture evaporates and mushrooms are browned. Stir in minced garlic for 1 minute.
Wilt Spinach: Add 10 oz fresh baby spinach in batches. Toss for 2 minutes until fully wilted and combined with chicken and mushrooms.
Creamy Base: Reduce heat to low. Stir in 1/2 cup Greek yogurt and 1/4 cup Parmesan. Mix until a smooth sauce forms. (Add 1 tbsp water if too thick).
Bake: Spread the mixture evenly in the skillet (or transfer to baking dish). Top with the remaining 1/4 cup Parmesan.