In a mixing bowl, combine the ground beef, chopped onion, breadcrumbs, beaten egg, 1/4 cup ketchup, Worcestershire sauce, salt, and black pepper. Mix only until the ingredients are evenly combined so the finished meatloaf stays tender rather than dense.

Shape the mixture into a compact oval loaf and place it in a microwave-safe shallow dish or ring-shaped casserole. Leave the center somewhat open or keep the loaf relatively thin to encourage more even cooking.

Cover the dish loosely with microwave-safe waxed paper, parchment, or a vented lid. Microwave on HIGH for about 6 minutes.

Carefully drain any accumulated fat from the dish, using oven mitts because the dish and liquid may be very hot. Rotate the dish if your microwave does not have a turntable, then continue cooking for 4–8 minutes.

Stir together the remaining ketchup, brown sugar, and yellow mustard. Spread the glaze over the meatloaf during the last few minutes of cooking, or apply it after the meatloaf is cooked and microwave for 30–60 seconds more.

Check the thickest part of the center with an instant-read thermometer, making sure the ground beef reaches 160°F (71°C). If it has not reached that temperature, continue microwaving in short intervals and check again.

Let the meatloaf rest for 5 minutes before slicing. This brief rest allows carryover cooking to finish the center and helps the slices hold together neatly.