Preheat the oven to 375°F (190°C) and position a rack in the middle.
Thaw the frozen spinach, then squeeze it in a clean kitchen towel or cheesecloth to remove as much liquid as possible.
In a large bowl, beat the softened cream cheese until smooth. Stir in the garlic powder, salt, black pepper, nutmeg, and cayenne (if using) until evenly combined.
Mix the milk into the cream cheese mixture to loosen it, then fold in the crumbled feta and the drained spinach until uniform.
Place the chicken breasts in a single layer in a large casserole or baking dish.
Spread the spinach and cheese mixture evenly over the top of each chicken breast, then sprinkle the grated mozzarella over the filling.
Bake on the middle rack for about 40–50 minutes, until the chicken reaches 165°F (74°C) internally and the topping is hot and lightly golden.
Remove from the oven, garnish with halved grape tomatoes if desired, and serve hot.
