In a medium skillet, melt 1 tablespoon butter over medium heat. Add the diced onions and saute for 25 to 30 minutes, stirring occasionally, until very soft. Side Note: Add a little water to the onions if they get dry while cooking.
Add in broth, a splash of wine and Worcestershire sauce. Cook for another 5 minutes, or until liquid is cooked down. Season with sea salt and freshly cracked pepper, set aside.
Preheat the oven to 325 degrees.
Melt 2 teaspoons of butter in a large cast iron skillet over medium heat. Throw in mushrooms and toss around for 2 minutes, just to start the cooking process. Season with sea salt and freshly cracked pepper, to taste.
Place mushroom caps face down in the skillet. Heap cavity with sauteed mushrooms, then sprinkle Gruyere over the top.
Bake for 10-11 minutes, or until cheese is melted and gooey.
Remove from the oven and sprinkle fresh parsley on top. Serve immediately. Enjoy.