Boil the pasta for 10-12 minutes until tender. Drain in a colander, run under cold water and set aside.
To make the pesto, put the coriander in a food processor with 2tbsp of oil and whizz to a rough paste. Add the walnuts and the rest of the oil and whizz again. Add 2tbsp of hot water and season.
Stir the pesto through the pasta until evenly mixed.
Carefully fold the red onion, tuna flakes and tomatoes into the pasta and serve immediately.