Make the Cioppino
In the pot, heat the olive oil until shimmering. Add the leek, fennel, celery, oregano and crushed red pepper and cook over moderately high heat, stirring, until the vegetables have softened, about 3 minutes. Pour in the tomato broth and bring to a simmer. Add the clams, cover and cook for 5 minutes, shaking the pot occasionally. Stir in the mussels and shrimp. Season the snapper with salt and place it on top of the stew; press gently to partially submerge it. Cover and cook until the fish is just white throughout, about 6 minutes.