Melt butter in a large sauté pan over medium heat.

Add mushrooms and shallots and cook until mixture is dry, stirring with a wooden spoon from time to time, 10 to 12 minutes.

The key to a flavorful duxelles is allowing all the juices to evaporate, thus concentrating the mushroom flavors.

The cooking time will vary depending on how much moisture is in the mushrooms. Take care not to brown the mushrooms or shallots.

When there are no more juices in the pan, add 1/2 cup of white wine and cook until liquid is once again evaporated, stirring from time to time, about 4 minutes longer.

Take from the heat and stir in a tablespoon of chopped parsley. Cover to keep warm.

Heat a pot large enough to contain all the mussels over high heat.

Add olive oil, mussels, remaining white wine and stock.

Season with salt and pepper and cover to steam the mussels.

The mussels will open up and turn bright orange in color when they are cooked.

Cook for about 6 minutes, shaking the pot from time to time to ensure even cooking, or until mussels are plumped and cooked through. Be careful not to overcook mussels as they tend to turn rubbery very fast.

Remove from heat. Discard any mussels that do not open up.

Allow to cool just enough so you can handle shells with your fingers.

Open shells and take mussels out, making sure to keep the smooth sides of the shells for serving (the flat bottoms of the shells will sit on a serving platter without wobbling).

To finish, mound a teaspoon or so of duxelles into a shell.

Top with a cooked mussel.

Repeat with remaining mussels and duxelles, setting the mussels on a serving plate.

Sprinkle with the remaining chopped parsley and serve with lemon wedges.