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 (1/2-cup each) 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.
Notes
Recipe Variations
Try sautéeing chopped green peppers and/or mushrooms with the onion.
Stick some cheese (cubes or shredded) in the middle for a gooey treat.
Skip the ketchup mixture and top your muffins with bacon strips that have been cut in half or thirds.
Try your favorite BBQ sauce in place of the ketchup mixture.
Shake things up by omitting the topping altogether and using mashed potatoes on top!
Spice things up and add a packet of taco seasoning to the ground beef mixture! Instead of the ketchup mixture, use your favorite salsa!
Watching your carb or sugar intake? Swap out the breadcrumbs for ground-up pork rinds. Use golden monk fruit sweetener (or similar product) in place of the brown sugar, and use sugar-free ketchup.