Heat olive oil in a skillet over medium heat, and cook and stir mushrooms and spinach until mushrooms are tender and spinach is wilted, about 3 minutes. Season to taste with salt and pepper; set aside.
Spread one side of each bread slice with butter. Place one bread slice, buttered side down, into the skillet. Top with the Swiss cheese, then spread the mushroom mixture on top of the cheese. Cover with the second slice of bread, buttered side up. Cook until the sandwich is golden brown on both sides, turning once. Cut in half and serve hot.
