In a medium skillet, melt 1 tablespoon of 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 the 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. Add the mushrooms and toss for 2 minutes. Season with sea salt and freshly cracked pepper to taste.
Place mushroom caps, top side down, in the skillet. Heap each cavity with sauteed mushrooms, then sprinkle Gruyere on top.
Bake for 10-11 minutes, or until cheese is melted and gooey.
Remove from the oven and top with some fresh parsley. Serve immediately. Enjoy.