Preheat your oven to 350°F (175°C). Ensure the rack is in the center position for even heat circulation around the muffin tin.

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

In a large bowl, combine the cooled cooked onions, ground beef, egg, milk, and dried bread crumbs. Season with salt and pepper to taste. Mix well using your hands or a sturdy spatula until the ingredients are just incorporated (do not over-mix).

Brush a 12-muffin tin (1/2-cup capacity each) with vegetable oil or olive oil.

Divide the ground beef mixture evenly between the muffin tin wells. Lightly pat the mixture into each well, ensuring a flat surface for the glaze.

In a separate small bowl, combine the brown sugar, mustard, and ketchup for the glaze. Stir until the sugar has dissolved and the glaze is smooth.

Bake the meatloaf muffins for 10 minutes. This initial phase sets the protein structure and prepares the surface for the glaze.

Remove the muffins from the oven and top each one with a generous spoonful of the ketchup mixture, spreading it evenly. The surface should be firm enough to support the glaze.

Return them to the oven and bake an additional 10 to 15 minutes, until the glaze has caramelized and the internal temperature reaches 165°F (74°C).

Let the muffins stand in the tin for 5 minutes before carefully removing them. This resting period is vital for the redistribution of internal juices.