Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine the ground beef with salt, pepper, chopped onion, and chopped bell pepper. Add the lightly beaten egg, diced tomatoes, quick-cooking oats, ketchup, brown sugar, and mustard. Mix gently to combine.

Transfer the mixture onto a clean surface and shape it into a loaf. Place it in a greased loaf pan.

In a small bowl, combine additional ketchup and brown sugar to create a glaze. Spread this mixture evenly over the top of the meatloaf.

Slide the loaf pan into the preheated oven and bake for approximately 1 hour, until browned and fully cooked (internal temperature of 160°F/70°C).

Let the meatloaf rest for about 10 minutes before slicing and serving.