Peel the lemons as you only want the skin. Chop the lemon rinds finely. Strain the lemon juice to remove any pips. Roughly chop the parsley, mint and garlic and blitz all these together in a food processor with a pinch of salt, chili flakes and the oil until smooth. Roughly chop the nuts, add them to the mix and blitz again until you have a rough pesto paste. Add half the cheese and give it a good stir. Taste for acidity and, if needed, add tablespoons of water to adjust. Bring a large pan of salted water to the boil and cook the ravioli for the time indicated on the packet. In the meantime, warm the butter in a large skillet together with the lemon pesto. Dilute with a ladle of hot pasta water to make it more of a sauce. Drain the ravioli, keeping some of the pasta water. Toss the ravioli with the warm pesto in the skillet, adding in the rest of the cheese and adding more pasta water if needed. Serve immediately.