How to Make It
In a large pot of boiling salted water, cook the pasta until al dente. Drain the pasta.
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.
Add the rigatoni to the sauce along with the parsley and cook for 1 minute, stirring to coat the pasta. Serve right away.