Preheat the oven to 350 degrees F (180 degrees C). Lightly grease a 9x5-inch loaf pan.

Heat oil in a large skillet over medium-high heat. Cook and stir onion, celery, and green pepper in hot oil until softened, about 5 minutes. Stir in ketchup to make a sauce. Remove from heat.

Combine 1/2 of the sauce with ground beef, bread crumbs, beaten egg, salt, and pepper in a large bowl; mix gently. Pat mixture into the prepared pan.

Bake in the preheated oven until an instant-read thermometer reads 160 degrees F (70 degrees C), about 1 hour. Remove from the oven and let rest.

Meanwhile, reheat remaining sauce over medium heat. Slice meatloaf and serve with warm sauce.