While the bread is toasting, heat a large frying pan on the stove over medium heat. Add a little oil, then sautee the green pepper, onion, and mushrooms until tender. Add the cream cheese and deli roast beef and stir until cream cheese is melted and distributed throughout. Spoon beef and vegetable mixture on top of toasted hoagie buns or bread. If using French bread, cut the bread into sandwich-sized pieces and serve.