Into simmering sauce layer your seafood in the following order: halibut, prawns, scallops, reserved clams, crab meat, clams/mussels. Put the lid on the pot and simmer about 10 minutes, or until clams/mussels have opened all the way. At this point all of the seafood should be cooked through and your Cioppino is ready to serve. Note: Do not stir the pot after you have layered in the seafood.