Preheat the oven to 375°F (190°C).
Spoon 1/2 cup of the tomatillo salsa into a 13x9-inch casserole dish and spread it to coat the bottom evenly.
Place the 4 chicken breasts in a single layer in the dish.
Season the chicken evenly with 1 teaspoon garlic powder, 1/2 teaspoon kosher salt, 1/4 teaspoon ground cumin, and 1/4 teaspoon ground black pepper.
Pour the remaining tomatillo salsa over the seasoned chicken, coating each piece.
Sprinkle 4 ounces shredded mozzarella evenly over the top.
Bake uncovered for 30–35 minutes, until the chicken reaches an internal temperature of 165°F (74°C).
Let the casserole rest a few minutes before serving.