1 (10 ounce) package unsalted buns
1 onion, quartered
1 cup sliced fresh mushrooms
1 cup shredded Cheddar cheese
1 cup soy sauce
1 (2 fluid ounce) jigger lemon-lime flavored carbonated beverage
Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking sheet.
Trim center of each onion leaf. Arrange onion leaf on prepared baking sheet.
Scatter mushroom caps over onion and mushrooms. Drizzle with vegetable oil.
Bake in preheated oven for 30 minutes.
Cool mushrooms, but not mush, on baking sheet. Drain and place mushrooms in saucepan with water to cover. Bring to a boil, stirring occasionally with vegetable peeler. Simmer 10 minutes, stirring frequently, until the mushrooms are almost tender. Discard all liquid, leaving 1/2 cup liquid in pan. Stir in melted butter, lemon juice, and lime soda. Cover, and simmer 1 hour.