Preheat the oven to 350F.
Remove the stems from the mushroom caps. Finely chop the stems.
Melt the butter in a large skillet. Add the onion and cook until translucent. Add the chopped stems and saute until soft.
Stir in the breadcrumbs, parmesan, seasonings and Worcestershire.
Pour the vermouth over, and use a fork to lightly toss the ingredients together.
Place the mushroom caps in a rimmed baking dish. Spoon the mixture into the caps. Bake in the preheated oven for 15 minutes. Serve warm.