Prepare the seafood: Slice the squid into rings, peel and devein the shrimp, and scrub the mussels or clams clean if you're using them.
Cook the pasta: In a large pot of salted water, bring it to a boil and cook the pasta according to the package instructions until al dente. Reserve 1 cup of pasta water before draining.
Sauté the aromatics: In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the chopped onion and sauté for about 3 minutes until softened. Add the garlic and cook for an additional 30 seconds until fragrant.
Add the seafood: Add the squid, shrimp, and optional mussels/clams to the pan. Sauté for about 3-4 minutes until the shrimp turn pink and the squid becomes tender.
Deglaze with wine: Pour in the white wine to deglaze the pan, scraping up any bits stuck to the bottom. Let the wine reduce for about 3 minutes.
Incorporate squid ink and stock: Stir in the squid ink and tomato paste. Add the fish stock and simmer for 3-4 minutes, allowing the sauce to thicken and flavors to blend.
Combine pasta and sauce: Add the cooked pasta to the skillet with the seafood sauce. If the sauce is too thick, add a bit of the reserved pasta water to loosen it up.
Garnish and serve: Plate the pasta, sprinkle with fresh parsley, and season with salt, pepper, and optional red pepper flakes for a little heat. Serve immediately and enjoy!