In a medium skillet, melt 1 tablespoon butter over medium heat. Add onions and saute 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. Throw in mushrooms and toss around for 2 minutes, just to start the cooking process. Sprinkle mushrooms with salt.
Place mushroom caps face down in a baking dish. Heap cavity with sauteed mushrooms, then sprinkle Gruyere over the top. Bake at 10 minutes on 325 degrees. Turn on broiler and broil for a couple of minutes, until the top of the Gruyere starts to bubble and slightly turn brown.