Bring 3 litres of water to the boil with 1 tbsp salt. Cook pasta per packet directions minus 1 minute. Reserve ~1 cup pasta water before draining.
In the same pot, heat reserved tuna oil over medium heat. Add garlic, anchovies, and chilli flakes. Cook and stir for 1 minute until fragrant.
Add drained pasta and capers to the pot with 1/3 cup reserved pasta water. Toss vigorously with 2 spatulas for 2 minutes, emulsifying the sauce.
Turn off the heat. Add drained tuna, parsley, lemon zest, and lemon juice. Season with salt and pepper to taste. Gently toss to combine, keeping some tuna chunks intact.
Serve immediately, garnished with extra parsley and lemon zest if desired.
Notes
Nutrition information is an estimate, calculated automatically. Values vary with brands, substitutions and portion sizes.