Dice the mushrooms into small pieces. Preheat an oiled skillet over medium-high heat. Sauté the mushrooms until golden brown.
Slice the onions into half rings. Add them into the skillet with the sautéed mushrooms and sauté them until golden brown. Season them with salt and set aside.
Cut the bread into 1/4-inch slices. Spoon the sautéed veggies on a slice of bread, followed by a handful of cheese. Place the other slice on top.
Preheat a skillet with butter over medium-low heat. Place the sandwiches on a skillet and turn the heat to low. As soon as the bread turns golden brown, flip the sandwich over and grill the other side.
Serve while the cheese is still melted and gooey. Enjoy!