slice the onion and saute in 2 T butter over medium heat until soft and golden brown, about 7-10 minutes, stirring occasionally. Stir in 1 c sliced mushrooms, stir 2-3 minutes more. Remove from heat.

Sprinkle onions and mushroom with sea salt and 1 t balsamic vinegar. Place slices of cheese on 4 pieces bread, top each with onion-mushroom mixture and another slice of bread. Cook on a griddle or skillet over medium heat until cheese is melted. 2-3 minutes per side.