Whisk egg, milk, salt, onion powder, and garlic-pepper seasoning together until pale yellow. Stir in Cheddar-Jack cheese until thick.
Spray skillet with cooking spray and cook egg mixture over medium-low heat, stirring until set.
Butter one side of each bread slice. Place one slice butter-side down in skillet, layer with mozzarella, scrambled egg, more mozzarella, and top with other bread slice butter-side up.
Cook until browned on both sides, flipping once. Let rest before serving.
