Reserve a cupful of the pasta cooking water when you drain the pasta. It will keep … Pour the pesto into a jar and cover with a little extra oil, then seal and store in the fridge. Just before draining, scoop out 1 cup of of the pasta cooking water. Return the drained pasta to the hot pan, and add a tablespoonful or so of pesto, adding your reserved pasta water a little bit at a time until the sauce reaches the desired consistency.