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Cook the pasta in a large pan of salted boiling water, adding the green beans and cherry tomatoes for the final 3 mins of cooking time. Drain, reserving a mug of cooking water.
Whizz the parsley, half the rocket, 3 tbsp of oil from the tuna, the lemon zest and juice, and some seasoning with enough of the reserved water in a food processor to form a spoonable pesto dressing.
Tip the pasta and veg back into the pan and add the pesto, both cans of tuna and the soured cream or cream cheese, stirring through until hot. Remove from the heat and toss the rest of the rocket through the pasta.