Heat a small frying pan to medium heat with butter. Add the mushrooms and red peppers and saute for 4-5 minutes until vegies are tender. Season with salt and pepper, to your liking. Set aside.
Meanwhile spread dijonnaise on one slice of rye bread. Place a slice of cheese on both breads (to help hold the mushrooms and peppers together). Now place the peppers and mushroom on top of cheese. Close sandwich and butter the outsides of both breads. Now place in a skillet and over medium heat cook until evenly brown and then flip. Cook until evenly brown on that side and enjoy!