The rouille, mussels and soup can be refrigerated separately overnight. Store the toasts in an airtight container. Recrisp in a 325° oven if necessary.
Jarred piquillo peppers, from the Navarre region of Spain, are available at specialty food shops. Piquillos are richer in flavor than roasted red bell peppers and have a bit of heat. The best substitute is jarred roasted red peppers plus 1/4 teaspoon of hot paprika.