In a medium skillet, melt 1 tablespoon butter over medium heat. Add onions and sauté for 15 to 20 minutes, stirring occasionally, until very soft. Splash in wine, broth, and Worcestershire. Cook for another 5 minutes, or until liquid is cooked down. Set aside.

Melt 1 tablespoon butter in a large skillet over medium heat. Add mushrooms and toss for 2 minutes to start the cooking process. Sprinkle mushrooms with salt.

Place mushroom caps face down in a baking dish. Fill cavity with sautéed onions, then sprinkle gruyère over the top. Bake for 10 minutes at 325°F. Turn on broiler and broil for a couple of minutes, until the top of the gruyère starts to bubble and slightly turn brown.

Sprinkle minced parsley over the top and serve.