Add the cream and, stirring constantly, bring it to a gentle simmer. (If your pasta is still a while from being done, remove the skillet from the heat, cover to keep warm, and then return to a gentle simmer before proceeding.) Add the lemon zest, lemon juice, chervil, and, if you feel it’s necessary, a very small ladleful of pasta water to the sauce and cook just until the sauce thickens, anywhere from 1 to 4 minutes.