Pasta with Vodka Sauce and Shrimp

Name: Pasta with Vodka Sauce and Shrimp
Ingredients: 1 small to medium onion; EVOO; 4 tablespoons butter; 3 large cloves garlic; Salt; 1 teaspoon Calabrian chili paste OR 1/2 teaspoon crushed red pepper flakes; 1 cup vodka; 1 can (28 ounces) Italian crushed tomatoes; A few leaves of basil, torn, plus a few to serve; 1 pound penne rigate pasta; 1 pound large raw shrimp, deveined and tails removed; 1 lemon; 1/4 cup Pernod or white vermouth; A handful flat-leaf parsley, finely chopped; 1 cup heavy cream; A handful freshly grated Parmigiano-Reggiano cheese, plus some to pass at table, optional ( -- and controversial to some Italians)