Melt butter in a large skillet over medium heat. Add onions, beef broth, worcestershire sauce, sugar, dried thyme, bay leaf, and a little salt and pepper. Bring the mixture to a boil over high heat then turn heat back down to medium and cook, stirring occasionally, until onions are soft and dark brown, 20 minutes. Cool slightly. Meanwhile, remove stems from mushrooms then place on a parchment paper or silpat lined baking sheet. Preheat oven to 350 degrees. Pour parmesan cheese into a small dish. In a large bowl stir together softened cream cheese, garlic powder, and pepper to taste. Add chopped prosciutto and onion mixture then mix until combined. Scoop by the Tablespoon (I used a cookie scoop) into mushroom caps - a little less for smaller mushrooms, a little more for larger mushrooms - then use a small spoon to make sure filling is packed into the cavity. Roll tops gently in parmesan cheese then place back on baking sheet and bake for 20-22 minutes or until tops are golden brown.