Preheat oven to 350°F.

Melt the butter in a medium skillet over medium-high heat. Add the chopped onions and cook, stirring occasionally, until the onions have softened. Remove from heat and let cool slightly.

In a large bowl, combine the cooked onions, beef, egg, milk, and bread crumbs. Season with salt and pepper to taste. Mix well.

Brush a 12-muffin tin with vegetable oil or extra-virgin olive oil. Divide the ground beef mixture between the muffin tin. Lightly pat it into each well.

In a separate small bowl, combine the brown sugar, mustard, and ketchup. Set aside.

Bake meatloaf muffins for 10 minutes. Remove meatloaf muffins from the oven and top with ketchup mixture. Bake an additional 10-15 minutes or until meat is no longer pink and a meat thermometer reads 160°F.

Let stand for 5 minutes before serving.