They sound and look elegant, but they are quick and easy to make. Prepare as instructed 
or add a sprinkle of Parmesan cheese just before popping them into the oven. 

Ingredients
12 Mussels, cooked/shucked
1 tbsp (15ml) PC unsalted butter, melted
2 tbsp (25 ml) lemon juice
1/8 tsp (0.5 ml) salt
12 large mushroon caps
 
Preheat oven to 450 F(230 C). Dip mussels in mixture of melted butter, 
lemon juice and salt. Place one mussel in each mushroom cap. Place in a 
lightly greased baking pan; pour remaining butter and lemon juice mixture 
over the mussels and mushroom caps. Bake in preheated oven for 10 
minutes. Serve hot.

Source:  peishellfish.com