Fish Soup with Mussels and Chorizo: Roasted Pepper Sauce

Char 1 large sweet pepper over an open flame or under the broiler until the skin is blistered and black all over. When it is cool enough to handle, scrape the skin from the flesh and remove the core and seeds. In a blender or food processor, puree the roasted pepper with 1 large tomato, roughly chopped, 1 garlic clove, coarsely chopped, ½