Bring salted water to boil in a large pot.
In a small bowl combine tuna, lemon juice, 1/4 tsp salt and 1/4 tsp black pepper.
Heat 2 tablespoons oil, 1 tablespoon garlic, anchovies, and red pepper flake in a saucepan over medium heat. Stirring occasionally while oil sizzles to break down anchovies. Cook for 1-2 minutes. Stir in tomatoes with juice and 1/2 tsp. salt. Increase heat high to bring to a strong simmer. Cook, stirring occasionally until thickened, adjusting heat to medium low. Keep warm on low while finishing.
Add spaghetti to salted water. Water should taste like the sea. Cook, stirring occasionally until pasta is cooked al dente. Reserve 1/2 cup pasta water. Drain pasta and return to pot.
Off the heat add tomato mixture, remaining salt and pepper, remaining garlic then toss until well combined. Add tuna mixture and toss gently. Cover and set aside for 3 minutes.
Adjust the sauce consistency with the reserved water. The reserved water works great for reheating the pasta the next day. Add 1/4 cup parsley and remaining oil then toss to combine. Season with salt and pepper to taste.
Drizzle with extra oil if wanted. Sprinkle with remaining parsley and serve.
