Stir the half & half and milk together in a large skillet. Bring the mixture to a rolling boil, stir in the grits and butter, then reduce the heat to medium low.
Let the grits simmer, stirring occasionally, covered for 5-7 minutes.
Stir the beaten egg into the grits mixture. Once evenly incorporated, stir in the cheeses. Continue stirring until the mixture's smooth, and the cheeses evenly incorporated.
Season the mixture with the garlic, and salt & pepper, to taste.
Lightly spray a 9x13 baking dish with non stick cooking spray. Transfer the mixture to the prepared dish. Use a spatula to smooth the mixture out evenly into the dish.
Bake at 350 degrees for 20-25 minutes, or until cooked through and golden brown on top.
Let the casserole rest 2-3 minutes before serving. Serve, as is, or with breakfast sausage, eggs, hash browns- your favorite breakfast fixins.