How to Make It
Preheat the oven to 425°. In a large ovenproof skillet, heat the olive oil. Add the carrot and bell pepper and cook over moderate heat, stirring occasionally, until lightly browned, about 8 minutes. Stir in the sliced scallions and tomatoes. Roast the vegetables in the oven until tender, about 10 minutes.
Scrape the vegetables into a food processor. Add the almonds, vinegar and pimentón and puree until smooth. Season the romesco with salt.
In an enameled cast-iron casserole, melt the butter. Thinly slice the reserved scallion greens and add to the casserole. Cook over moderate heat, stirring, until softened, 1 minute. Add the mussels, Cava, clam juice, cream and 1/2 cup of the carrot romesco and bring to a simmer. Cover and cook until the mussels open, about 3 minutes. Discard any that don’t open. Season with salt and stir in the parsley. Serve the mussels with the toasts and the remaining romesco.