Preheat the oven to 430F (220C);

Meanwhile, cook the mussels  "al Vapor" by heating them in a pan with a tight lid along with some olive oil and 2 cloves of sliced garlic . Take them out as soon as each one opens. Allow them to cool down, and remove one of the two shells of each mussel

Use a sharp knife to chop the garlic very fine, in this way you will get small dry particles that will blend well with the other dry ingredients of the gratin

Make the gratin by mixing the chopped garlic, parsley and bread crumbs

Arrange the mussels in a shallow dish and sprinkle some of the gratin over each of the mussels

this dish is ready to go into the oven

Sprinkle some olive oil and some of the cooking liquid over each of the mussels

Place the dish with the mussels in the preheated oven and allow them to cook for some 8 minutes

Allow the dish with the mussels to cool down a bit before you serve them 

Sprinkle some lemon juice over the mussels;

Source: tapasbonitas.com