In a large bowl, combine lean ground beef, tomato sauce, quick-cooking oats, beaten eggs, chopped onion, chopped bell pepper, garlic powder, dry mustard, salt, and black pepper. Use your hands to gently mix until all ingredients are evenly combined.
Divide the mixture into 4 equal parts. Form each portion into a patty and press lightly in the center to create a small dimple so they cook evenly.
Heat a nonstick skillet over medium heat and add a small amount of oil. Cook the patties for about 5-7 minutes on each side until nicely browned and the internal temperature reaches 160°F. Allow the patties to rest for 3 minutes before serving on buns with your desired toppings.