Meanwhile, in a large, deep skillet, heat the olive oil until shimmering. Add the onions and garlic and cook over moderate heat until softened, about 5 minutes. Add the crushed red pepper and tomatoes with their juices and bring to a boil. Cook over moderately high heat, stirring occasionally, until the sauce is very thick, about 8 minutes. Add the clam juice and simmer until the sauce is thickened and slightly reduced, about 5 minutes. Season the fish with salt and stir it into the sauce. Simmer for 2 minutes, until the fish is nearly cooked through.