Rinse and drain mussels. Check for any cracked shells and discard. Also check to ensure all shells are tightly closed. Gently tap any that re open and discard if they don't close. Remove any beards by pulling strings back and forth until they release.
Finely mince garlic and onion. Saute in butter over medium heat until softened. Squeeze juice of half a lemon over top and add white wine.
Turn burner to high. Add mussels to pan and cover tightly with a lid. Steam for about 4-5 minutes without removing the lid. Check to see if mussels have opened wide. If not, steam another minute or so. Do not over cook.
Remove pan from heat. Stir well to combine juices and get bits of onion and garlic into the opening of the mussels.
Discard any mussels that have not opened.
Sprinkle fresh chopped parsley over top. Serve with crusty bread for dipping.