Heat the oven to 350 °F before preparing the meat mixture. Starting with a fully heated oven helps the small loaves cook evenly from the beginning.

Stir the brown sugar, ketchup, Dijon mustard, and nutmeg together in a small bowl. The glaze should look smooth and thick enough to coat the back of a spoon.

Combine the ground beef, panko, chopped onion, barbecue sauce, mustard, garlic powder, chili powder, salt, pepper, and lightly beaten egg in a large bowl. Use clean hands, a meat chopper, or a large spoon and stop mixing as soon as everything is evenly combined.

Divide the mixture among the 12 muffin cups and shape each portion with a gentle rounded top. A light dome keeps the meat tender and leaves room for rendered grease around the outer edge.

Bake the portions for 15 minutes, then remove the pan and blot visible grease from the tops with paper towels. The tops should look set but will not yet be fully cooked through.

Spoon the glaze generously over each portion and return the pan to the oven for 10 to 15 minutes. The glaze should look shiny and lightly set when the meatloaves are ready.

Let the mini meatloaves sit briefly in the pan before lifting them out and serving. The resting time helps the juices settle so the portions stay moist when cut.