Whisk the milk, heavy cream, salt, pepper, and garlic powder into the roux until no lumps remain. Turn the heat up to medium-high, and bring to a gentle boil to thicken for 2-3 minutes. Add a splash of milk as needed to thin it out. Turn the heat back down and add each kind of cheese, continuing to stir until it's melted.
2 1/2 cups whole milk, 1 cup heavy cream, 1 1/2 cups (6 ounces) shredded sharp white cheddar cheese, 1 1/2 cups (6 ounces) gruyere, manchego, or fontina cheese, 1 teaspoon kosher salt, 1/2 teaspoon garlic powder, Pinch of black pepper