1/2 cup vegetable oil, divided
2 large onions, halved and seeded
8 slices Swiss cheese
salt and pepper to taste
8 hamburger buns
Heat vegetable oil in a large pot over medium heat.
Place onions in oil and cook about 3 minutes, turning once, until brown and transparent. Stir in cheese. Cook 5 minutes, stirring constantly.
When cooled, remove from oil and brown in skillet with onion oil. Stir in salt and pepper. Place mushrooms into buns, and toast each side until browned. Serve at once or refrigerate in sauce.