Preheat oven to 350 degrees F (175 degrees C). Lightly grease 9x13-inch baking pan.
Melt butter in a saucepan over medium heat; saute onion and garlic until onion is translucent, 5 to 10 minutes. Add green bell pepper to onion mixture and saute until bell pepper is softened, about 3 more minutes.
Mix sirloin, bread crumbs, salt, and black pepper together in a large bowl with your hands; add vegetable mixture and loosely mix. Mix egg into sirloin mixture; add oats if mixture is too wet. Transfer sirloin mixture to the prepared baking pan and form into a loaf.
Bake in the preheated oven for 20 minutes. Whisk ketchup, sugar, honey, and hot sauce together in a bowl; brush over meat loaf. Continue baking until no longer pink in the center, about 40 more minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C). Let stand for 10 minutes before slicing.