Fill a large pot with water, cover it, and bring it to a boil over high heat.

Set a colander in the sink and place a large heatproof serving bowl on the counter near the stove.

Put the ricotta into the serving bowl.

Add the pesto to the bowl with the ricotta.

Add 1/2 cup of the grated Parmesan to the bowl, saving the rest for serving.

Add the lemon zest, 1 tablespoon of lemon juice, and the black pepper to the bowl.

Stir the mixture with a spoon until it looks mostly smooth. It may seem thick now, and that is fine.

When the water boils, add 1 teaspoon of the salt to the pot.

Add the pasta shells to the boiling water and stir well once so they do not stick together.

Cook the shells according to the package directions until they are tender but still a little firm in the center.

Two minutes before the pasta is done, add the frozen peas to the pot with the shells.

Scoop 3/4 cup of the pasta cooking water into a mug or measuring cup before you drain the pot.