Preheat the oven to 375°F (190°C). Lightly grease a standard loaf pan and set aside.
Prepare the meatloaf mixture
In a large mixing bowl, combine the ground beef, diced tomatoes (without juice), onion, green pepper, quick oats, salt, and black pepper. Mix thoroughly until all ingredients are evenly incorporated.
Shape and place in pan
Form the mixture into a loaf shape and transfer it into the prepared loaf pan, pressing lightly to ensure even cooking.
Prepare the glaze
In a small bowl, whisk together the ketchup, brown sugar, and mustard until smooth. Spread this glaze evenly over the top of the meatloaf.
Bake
Place the loaf pan in the preheated oven and bake for 55–60 minutes, or until the internal temperature reaches 160°F (71°C). Allow the meatloaf to rest for several minutes before slicing and serving.
Notes
To prepare this dish gluten-free, substitute the quick oats with certified gluten-free oats or use gluten-free breadcrumbs. Verify that the mustard and ketchup used are labeled gluten-free.