Melt some butter in a large sauté pan and add the frozen mussels. Place a lid over the pan and steam until all the mussels are open, shaking the pan occasionally. Note: discard any that will not open. 

Remove the lid and add the cream, garlic, and lemon juice, and continue to cook until the sauce starts to thicken. Test for seasoning and add salt and pepper as required.

Transfer the mussels and the broth to a serving bowl, and sprinkle with chopped parsley. Serve immediately.