Wash cockles and mussels thoroughly in running water, removing beards.
In a large pot, melt the butter, then add the finely-chopped onion. Cook over medium heat until onion is translucent. Add garlic when onion just becomes soft. Do not brown.
Stir in parsley and wine. Season with salt and pepper to taste. Bring liquid to a boil, then lower heat so that liquid is simmering only.
Add mussels to the pot. Put cockles on top of mussels. Squeeze in a bit of lemon juice, and cook until shells open.
When shells are open, remove from burner and put in serving bowl. Do NOT cook until all shells are opened. Some may not open. Discard any that don’t open.
Serves: 4 – 6