Start by scrubbing the mussels under cold running water to remove any sand or debris, using a brush if necessary. Remove the beards by pulling them towards the hinge of the mussel. Discard any mussels that are cracked or remain open after tapping lightly.

In a large pot, add ¼ cup of water and bring it to a gentle boil over medium heat. Add the cleaned mussels, cover the pot, and steam them for about 3-4 minutes until the shells open up wide. Discard any mussels that fail to open during cooking.

While the mussels are steaming, combine softened unsalted butter, minced garlic, grated Parmesan cheese, chopped parsley, lemon zest, salt, and pepper in a mixing bowl. Blend everything well until you achieve a creamy mixture.

Carefully place the steamed mussels on a baking sheet, loosening them from their bottom shells with a spoon. Discard the empty top shells.

Spoon the prepared garlic butter mixture generously over each mussel. Turn your oven broiler to high and place the baking sheet under the broiler for about 2-4 minutes until the tops are golden brown and bubbly.

Remove the baking sheet from the oven and garnish with extra chopped parsley. Serve immediately on a platter alongside crusty bread.