Rouille is a garlicky, saffron-and-cayenne sauce from Provence, France, traditionally served alongside bouillabaisse and other seafood soups. It's made by emulsifying olive oil with breadcrumbs or potato, garlic, saffron, and cayenne pepper into a thick, rust-colored, spicy result. It's often confused with aioli: both are Provençal olive-oil emulsions, but rouille adds saffron, chile, and a starchy thickener that aioli lacks. Beyond bouillabaisse, it's spread on the grilled bread that floats in the soup, or used as a dip or spread for grilled fish and vegetables.