A staple in coastal cuisines across the globe, seafood-based soup can take many forms. Often, it was employed as a means to cook up less popular fish. In Southern France, bouillabaisse combines a few types of fish simmered over the stove. Our version follows more from deep-red Hungarian fisherman’s soup, which got its color from tomatoes and paprika. The recipe relies on halibut fillets and mussels, both widely available at grocery stores, and gets its complex flavor from leeks, fennel, diced tomatoes, and crushed red pepper. Serve with mixed olives for a flavorful accompaniment that evokes crashing waves and salty sea air.