Preheat oven to 375°F. Mix ground beef, egg, mustard, BBQ sauce, garlic powder, pepper, onion, and breadcrumbs in a bowl.
Shape into a loaf, place in a pan, brush with BBQ sauce, cover with foil, and bake for 45 minutes or until internal temp hits 160°F.
Let the meatloaf rest for 20 minutes, then cut into ¾-inch slices.
Butter bread slices. In a skillet, layer bread (butter-side down), cheese, meatloaf, more cheese, and another bread slice (butter-side up).
Cook 3–4 minutes per side over medium heat until golden and cheese is melted.