Place the mussels in a pan with 3/4 cup of water and the lemon slice. Bring to a boil and remove mussels as they open. Do not overcook. Reserve 1/2 cup of the cooking liquid. Mince the mussell meat. Separate the shells and discard 1/2 of them. Heat the olive oil in a small skillet. Add the onion and sauté until it has wilted. Stir in the tomato paste , the minced mussel meat, cilantro, lemon juice, salt & pepper. Cook for 5 minutes. Half fill the shells with this mixture and set aside.

WHITE SAUCE

Melt the butter in a saucepan over moderate heat. Add the flour and stir a minute or two. Gradually pour in the reserved mussel broth and the milk (at a ratio of one to one) until sauce is thickened and smooth(it will be quite thick). Season with salt & pepper. Remove the pan from the heat and cool, stirring occasionally.

FINAL PREPARATION

Using a teaspoon, cover the filled mussel shells with the white sauce, sealing the edges by smoothing with the back of a spoon. Refrigerate for one hour or more until the sauce hardens. Beat the eggs in a shallow bowl. Mix together the crumbs and the cheese. Dip the mussels into the egg, then into the crumb mixture. Heat the oil, at least one inch deep, in a skillet. Fry the mussels, fried side down, until they are well browned. Drain. the mussels can be kept in a 200 degree oven for up to 30 minutes.