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 evenly to coat the bottom.
Place the chicken breasts in the prepared dish in a single layer.
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, covering it, then sprinkle the shredded mozzarella evenly on top.
Bake for 30–35 minutes, until the chicken reaches an internal temperature of 165°F (74°C) and the cheese is melted and bubbly.
Let the casserole rest a few minutes before serving.