Preheat your oven to 350°F (175°C). Grease your loaf pan.

In a large mixing bowl, combine ground beef, salt, black pepper, chopped onion, chopped bell pepper, egg, canned diced tomatoes, and quick-cooking oats. Mix until just combined.

Transfer the mixture into the prepared loaf pan, shaping it into a loaf.

In a separate bowl, stir together the ketchup, brown sugar, and mustard. Spread half of the glaze over the top of the shaped meatloaf.

Bake in the preheated oven for about 1 hour, until it reaches an internal temperature of 160°F (70°C).

Remove from the oven, spread the remaining glaze on top, and broil for 2-3 minutes.

Let the meatloaf rest for 5-10 minutes before slicing.