Preheat oven to 375 degrees F (190 degrees C). Grease a 9x5-inch loaf pan.
Combine beef, 2 cups ketchup, onion, stuffing mix, and egg in a large bowl using your hands.
Mix 1/2 cup ketchup and mustard powder in a small bowl until glaze is smooth.
Transfer beef mixture to the greased loaf pan. Brush the top with the glaze mixture. Sprinkle brown sugar on top of the glaze.
Bake in preheated oven until no longer pink in the center, 35 to 40 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).