"It is rare for me to come up with a recipe that is just too good not to write down. Usually, when I cook, I like to play around with the ingredients and so I rarely make something the same way twice. But this meatloaf was as close to perfect as I could imagine. I tried to be specific in my directions but there is no way to determine ahead of time how wet the mixture will be after adding the vegetables. How wet is too wet is hard to describe, but it is better to err on the side of moisture, I'd think."
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.