Preheat your oven to 375°F. Spray a 13x9-inch baking dish with nonstick spray and set it aside

In a large bowl, combine the cooked shredded chicken, cream of chicken soup, mixed frozen vegetables, minced garlic, ground black pepper, shredded mozzarella cheese, and grated mild cheddar cheese. Stir everything together until the mixture is well combined and evenly distributed

Spread the chicken and vegetable mixture evenly into the prepared baking dish. Make sure the top is level so the biscuits will bake evenly later

Open the canned biscuits and cut each biscuit into quarters. Place all biscuit pieces in a large bowl. Drizzle the melted butter over the biscuit pieces and toss them to coat evenly

Arrange the buttered biscuit pieces on a lightly greased baking sheet. Bake in the preheated oven for 5 to 7 minutes until they begin to puff up and just start to brown

Carefully transfer the par-baked biscuit pieces and arrange them evenly over the chicken mixture in the baking dish. Place the dish back in the oven and bake, uncovered, for 20 to 25 minutes, or until the biscuits are golden brown and the casserole is bubbly

Let the casserole cool for a few minutes before serving