Preheat the oven to 375 degrees and lightly grease a baking dish that will hold the peppers upright.
Cut the tops off the bell peppers and remove the seeds and membranes. Set the peppers aside.
In a mixing bowl, combine the ground beef, crushed Ritz crackers, diced onion, egg, milk, salt, and black pepper. Mix until just combined.
Spoon the meatloaf mixture evenly into the four prepared bell peppers, pressing gently to fill each one.
Place the stuffed peppers in the baking dish. In a small bowl, stir together the ketchup, brown sugar, and yellow mustard until smooth.
Spoon the glaze over the top of each stuffed pepper, letting some run down the sides.
Bake for 35 to 40 minutes until the peppers are tender and the internal temperature of the meatloaf reaches 160 degrees. Let the peppers rest for 5 minutes before serving.