Cook pasta and peas: Bring a large pot of water to a boil, add salt generously and cook according to the packet’s instructions until al dente, adding the frozen peas during the last 2-3 minutes of the cooking time. Reserve some pasta water and drain the rest.
Prepare ingredients: Wash, dry, and zest the lemon. Squeeze the juice to get about 5-6 tablespoons. Grate the Parmesan. Set both aside.
Mix sauce: Whisk ricotta, heavy cream, Parmesan, lemon zest and juice, olive oil, pepper flakes or cayenne pepper, about ½ teaspoon salt, and about ⅛ teaspoon ground pepper. Add about ¼ cup of the pasta water and whisk well.
Combine: Return the pasta and peas to the pot. Pour the ricotta sauce on top. Add a little more reserved cooking water if necessary to thin the sauce to your liking.
Adjust the taste with more salt and freshly ground black pepper. Serve immediately.
Notes
Spaghetti or any other pasta, short or long, is good.
You will need 1 tablespoon lemon zest and about 5-6 tablespoons of lemon juice. One large or 1 ½ medium lemon should do it.