1 pound ground sweet Italian sausage
1 small onion, peeled and chopped fine
3/4 teaspoon salt
1/4 teaspoon pepper
1/8 to 1/4 teaspoon red pepper flakes (optional)
1/2 cup plus 3 1/2 cups chicken broth, measured separately
4 1/2 cups medium pasta shells (12 ounces)
2 cups frozen peas
1/2 cup grated Parmesan cheese (1 ounce), plus extra for serving
2 teaspoons grated lemon zest plus 1 tablespoon lemon juice, zested and squeezed from 1 lemon
1. Add sausage, onion, salt, pepper and pepper flakes (if using) to a Dutch oven. Use a wooden spoon to break sausage into small pieces. Cook over medium-high heat, stirring occasionally and continuing to break up sausage, until sausage is lightly browned and dark brown bits have formed on bottom of pot, 10 to 12 minutes.
2. Carefully add 1/2 cup chicken broth and use a wooden spoon to scrape up browned bits on bottom of the pot. Cook until liquid has mostly evaporated, about 2 minutes.
3. Stir in pasta and remaining 3 1/2 cups broth. Bring mixture to a boil.
4. Reduce heat to medium-low and cover pot with lid. Cook until pasta is tender, 10 to 12 minutes. (Some liquid will remain in pot.) Turn off heat.
5. Add peas, Parmesan, lemon zest and lemon juice. Use a wooden spoon to stir vigorously for 1 minute. Let sit, uncovered, for 5 minutes to allow peas to warm through and sauce to thicken slightly. (Sauce will continue to thicken as pasta cools.)
6. Use a ladle to divide pasta among individual bowls. Sprinkle with extra Parmesan cheese, if desired. Serve.