Place a large saucepan of salted water over a high heat and bring to the boil. Once the water is boiling, stir in the pasta and cook for 10 minutes.
When the pasta is cooked, drain and return to the pot with two tablespoons of reserved pasta cooking water.
Place a frying pan over medium heat and add the tuna (in its oil), along with the sliced garlic and cook for five minutes.
Stir in the capers and chilli flakes, and cook for another minute. Remove from heat and stir in the lemon zest and juice, one tablespoon of olive oil, and parsley. Season with freshly ground black pepper.
Stir the tuna and mix into the spaghetti, along with another tablespoon of olive oil and the chopped fresh flat-leaf parsley.