I had bought 1 lb of deli roastbeef on sale and, with a hungry family of 5, I had to figure out a way for it to feed us. I decided to fry up the meat and add the onions and green peppers to make sandwiches. It was a hit with all of us and is a requested item when we get together! I don't recommend using anything but butter, BUT if you need to substitute, olive oil would be a good idea so as to enhance the flavor of this recipe as the butter does.
Using a med frying pan or griddle, heat 1 pat butter over med high heat. When the butter is melted, add 1/4 of each onions and green peppers. Add salt to taste and saute stirring fairly often til cooked through, but not soggy. Transfer to a container, cover, and continue to cook rest of onions and peppers in like manner. If the pan you're using gets burned on the bottom, clean the pan and reduce the heat as you don't want the burnt flavor ruining your dish!
Take the butter and melt it in the same pan you sauted onions and peppers or use another frying pan (if you can multi-task, cooking meat in another pan while onions and peppers are cooking makes the process SO much quicker!) saute about 1/4 of the roast beef adding one clove worth of garlic and salt to taste. Transfer to container, cover and continue to cook rest of meat in like manner.
When ingredients are ready, remove bread from oven, slice in half length-wise and put meat, cheese, onion and pepper mix on it. Serve with a side salad (if you want) and enjoy!