Method:
Bring a large pot of salted water to a boil. Add pasta and cook until al dente, 8 to 10 minutes. Drain well.
Meanwhile, heat olive oil in a large skillet. Add onion and garlic and cook until fragrant and starting to turn translucent, 3 to 5 minutes. Add red pepper flakes and tomatoes with their liquid. Bring to a boil, reduce to a simmer and cook 8 to 10 minutes. Add clams, 3 tablespoons of the reserved clam liquid and black pepper. Simmer until sauce is slightly thickened, about 5 minutes. Add more red pepper and clam liquid, if desired. Stir in chopped parsley and pasta. Toss to coat pasta with sauce and serve immediately.