Preheat your oven to 350°F.
Start with the sauce. In a small bowl, stir together the brown sugar, ketchup, Dijon mustard, and nutmeg until smooth. Set it aside.
In a large mixing bowl, combine the ground beef or turkey, breadcrumbs, chopped onion, barbecue sauce, mustard, garlic powder, chili powder, salt, black pepper, and beaten egg.
Mix everything until evenly combined. You can use your hands, a spoon, or a meat masher.
Divide the mixture evenly into a 12-cup muffin pan. Press the meat mixture down gently so each cup is filled evenly.
Place the pan in the oven and bake for 15 minutes.
Remove the pan from the oven and carefully blot away any excess grease from the tops using paper towels.
Spoon the prepared sauce generously over each mini meatloaf.
Return the pan to the oven and continue baking for another 10 to 15 minutes, or until the meat is fully cooked through.
Serve warm with mashed potatoes and extra sauce on the side.