Pasta e Fagiolini

Name: Pasta e Fagiolini
Ingredients: 1 medium white or yellow onion, thinly sliced; 1 head of garlic, cloves separated, coarsely chopped; ¼ cup extra-virgin olive oil, plus more for drizzling; 1 Tbsp. Diamond Crystal or 1¾ tsp. Morton kosher salt, plus more; 1 lb. green beans, trimmed, cut into 2 pieces; 1 28-oz. can crushed tomatoes; 2 Tbsp. unsalted butter, divided; 1 tsp. crushed red pepper flakes; 12 oz. medium tube-shaped pasta (such as rigatoni or penne); Zest of 1 large lemon; 2 oz. Parmesan, finely grated (about 1 cup), plus more for serving; 1 Tbsp. fresh lemon juice; Coarsely chopped mint and/or parsley and lemon wedges (for serving)