Preheat your oven to 375°F (190°C). Line a baking tray with foil and lightly grease it with cooking spray. This helps with easy cleanup later.
In a large bowl, combine the salt, pepper, chopped onion, chopped bell pepper, beaten egg, drained diced tomatoes, and quick-cooking oats. Mix until well combined.
Gently add the ground beef to the mixture and combine until everything is evenly mixed.
Transfer the mixture onto the prepared baking sheet, shaping it into a loaf.
For the topping, mix together ketchup, brown sugar, and mustard in a small bowl. Spread this mixture generously over the top of the meatloaf.
Bake the meatloaf in the preheated oven for 1 hour. Use a meat thermometer to check that it reaches an internal temperature of 160°F (71°C).
Once done, remove the meatloaf from the oven and allow it to rest for about 10 minutes.
Notes
For added flavor, consider mixing in Worcestershire sauce or garlic powder to the meatloaf mixture.