Preheat oven to 400 degrees. Slice deli roast beef into stripes. Slice bell peppers down the middle. Clean out the insides and place in a casserole dish.
Over medium heat warm a skillet with oil. Add onions and then roast beef. Season with Worcestershire sauce then salt and pepper to taste. Cook just until warmed. Add to diced bell peppers.
Top bell peppers with cheese and place in the oven for 15-20 minutes. Until cheese is melted and peppers are tender.
Cooking Tip: Don't overstuff the peppers as you want the cheese to be able to melt down into the peppers.Do you have to pre-cook your peppers?No, you do not have to pre-cook or boil your peppers before you fill and bake them. I like my stuffed peppers to be crunchier and crispier, so I do not pre-cook them or bake for a long time. I love that crunch the fresh bell peppers still have with only baking for around 20 minutes.Some people do what their stuffed peppers to be softer and not as crisp, so I would recommend pre-cooking them by boiling. You would want to fill a pot with water and fill your peppers with water as will. Cut the top off, and fill with water, this will help keep them submerged in the boiling process. Bring your peppers to a boil, and then reduce heat to medium and allow the peppers to simmer for about 3-5 minutes. Any time longer will really make them soft. Once they are boiled, put in an ice bath to cool rapidly for the remaining process. Slice them, stuff them, then bake them as before. This will result in a softer pepper once done.Cooking Tip:I slice mine in half and fill because I find it easier to eat. You can even skip the fork and pick it up and eat it just like you would a sandwich.