Philly Cheesesteak Sandwiches

1 lb. thinly sliced deli roast beef
1 medium onion, diced
1 green pepper, diced
2 tblsp. butter
sliced swiss cheese
hoagie buns
mayonnaise
mushrooms, sliced (optional)

Melt butter in medium frying pan. Add onions and peppers and cook over medium heat until tender. Add roast beef and cook until warm. Reduce heat to low and place one layer of swiss cheese over the top. Meanwhile toast the buns in the oven.

Spread mayo on the buns and top with everything else. Serve immediately.