Slice rolls or bread in half and butter both outsides with butter. Place one buttered side on top of the other and set aside. Heat Panini maker.
Meanwhile, heat oil in a sauté pan and add peppers, onions and mushrooms when hot. Sauté about 5 minutes, until tender. Remove from heat and set aside.
On sliced bread with open side up, layer spinach, sautéed peppers and mushrooms and cheese. Sprinkle cheese with season salt.
Place buttered side down of layers sandwich on hot Panini maker and top with remaining buttered bread/roll. Close lid to Panini maker and grill about 3-4 minutes until cheese is slightly melted.
Remove from Panini maker, slice and serve.