• 8 little red potatoes
• ½ cup plus 2 tablespoons olive oil
• Salt and freshly ground pepper
• 2 cups diced onions
• 1⅓ cups dry white wine
• 6 pounds mussels (about 15 per serving), scrubbed
• 1 cup toasted bread crumbs
• 4 teaspoons finely minced garlic
• ½ cup chopped fresh parsley
• ½ cup freshly grated pecorino or Parmesan cheese

Preheat the oven to 375°F. Rub the potatoes with 2 tablespoons olive oil and sprinkle with salt and pepper. Roast in a baking dish until tender but still firm enough to slice, about 25 to 30 minutes.

Heat ¼ cup oil in a large skillet over medium heat. Add the onions and cook until tender, 5 to 7 minutes.

Pour the wine into a wide saucepan, cover, and heat to simmering. Drop in the mussels and steam covered until they open. Using a slotted spoon, transfer the mussels to a colander. Let stand until cool enough to handle. Strain the liquid through a cheesecloth-lined strainer. Carefully remove the beards from the mussels and remove them from the shells. Add the mussels to the cooking liquid and refrigerate.

Preheat the oven to 350°F. Oil 4 individual gratin dishes, about 5 inches in diameter.

Slice the potatoes about ⅓ inch thick and toss with ¼ cup olive oil. Sprinkle lightly with salt and pepper. Layer them in the gratin dishes and top with the cooked onions. Add the mussels and drizzle all with about 2 tablespoons of the liquid. Combine the bread crumbs, garlic, parsley, and cheese and sprinkle over the top of the gratins.

Bake until bubbly and the tops are browned, 10 to 12 minutes.