Slice mushrooms and onions. Toast bread lightly before grilling. Coat skillet with olive oil. Place sliced mushrooms and onions in skillet, cooking until golden brown (about 6-8 minutes). Set aside.
Butter both sides of bread and place in skillet. On each side of bread, add 1-1 ½ slices of cheese, keeping both sides open. After 3-5 minutes on medium heat, add portion of pre-cooked mushrooms and onions and combine the sandwich. Slice in half and serve warm. Repeat and enjoy!