Melted Fennel Pasta

Name: Melted Fennel Pasta
Ingredients: 3 large fennel bulbs with fronds (about 3 lb. total); ¼ cup extra-virgin olive oil; 1 medium onion, thinly sliced; 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, plus more; 1 tsp. sugar; 4 garlic cloves, finely chopped; 3 oil-packed anchovy fillets; ½ tsp. crushed red pepper flakes; 12 oz. gemelli, fusilli, or other medium pasta; 1 oz. Parmesan, finely grated (about ½ cup); 2 Tbsp. unsalted butter; 1 tsp. finely grated lemon zest; 2 Tbsp. fresh lemon juice