Mix tomato sauce, brown sugar, and mustard in a bowl until sugar dissolves.

Mix onion, cracker crumbs, bell pepper, eggs, salt, pepper, and garlic powder in a large mixing bowl until well combined. Add ground beef and 1/2 of the tomato sauce mixture; stir until thoroughly combined. Transfer to a 2-quart, microwave-safe baking dish. Spread remaining tomato sauce mixture over top.

Microwave on high until the loaf is set, the juices run clear, and meat is no longer pink inside, 10 to 15 minutes depending on the power of your microwave. An instant-read meat thermometer, inserted into the center of the loaf, should read 165 degrees F (75 degrees C).

Remove from the microwave and immediately drain any grease from the dish. Let stand, uncovered, 10 to 15 minutes before serving.