Bring a big pot of water to a boil and add the pasta; cook until al dente, adding the peas for the last 4 minutes of cooking time. Before you drain it, scoop out some of the water with a measuring cup.

Drain the pasta and peas and put it back in the pan - add a knob of butter, a squeeze of lemon and lots of freshly grated Parmesan cheese, and enough of the pasta water to make a saucy consistency.