A rouille is a Provençal sauce that takes its name from the French word "rust." This is due to its ruddy saffron or chili-induced color.

Something along the lines of a kicked-up eggless mayonnaise, rouille is made from pounded garlic and chilies or saffron that is mixed with breadcrumbs and emulsified with fish stock and sometimes a touch of olive oil until thickened and increased in volume.

Rouille is smeared on baguette toasts and floated in bouillabaisse, and can also accompany poached fish. Depending on the cook, sometimes rouille is embellished with fish liver or lemon juice.