Heat skillet over medium heat, add butter and saute onions until they are clear. Combine beef, egg, onion, green pepper, oats and half of the tomatoes and mix well. Season with salt and pepper and shape into a loaf in a glass baking dish. Place the strip of bacon lengthwise on the top of the loaf and bake, covered, at 350 F for 45 minutes. Remove, pour remaining tomatoes over the top, return to oven and bake uncovered for 30-45 minutes until cooked through.