Preheat your oven to 375°F.
Prepare a baking dish by lining it with foil and spraying it lightly with cooking spray.
In a large bowl, combine the ground beef, onion, bell pepper, egg, oats, tomatoes, salt, and black pepper.
Mix gently to form a cohesive mixture taking care not to overmix.
Shape the meat mixture into a loaf and place it in your prepared baking dish.
Slightly flatten the top to help the topping stick and cook evenly.
In a small bowl, mix together the ketchup, brown sugar, and Dijon mustard.
Spread this mixture evenly over the top of the meatloaf, using a spoon or spatula.
Bake in your preheated oven for about 1 hour or until fully cooked.
Allow the meatloaf to rest for a few minutes before slicing and serving.