In a medium skillet, over medium heat, melt 1 tablespoon of butter and add the onion, cooking for 15-20 minutes until the onions are very soft, stirring occasionally. Add the splash of wine, Worcestershire sauce, and beef broth and cook for another 5 minutes until the liquid is cooked down. Set aside.
Melt the other tablespoon of butter and toss in the mushrooms for a couple of minutes, just to start the cooking process.
Place the mushroom on a baking try and fill with the onions. Sprinkle the top with Swiss cheese. Bake on 325 for 10 minutes. Broil for a couple of additional minutes until cheese starts to bubble and starts to turn brown. Sprinkle with parsley and serve.