Extra basil and Parmesan for garnish
¼ cup finely grated Parmesan cheese
2 tbsp. unsalted butter cut into pieces
12 oz. spaghetti
1/4 cup chopped basil
1/2 tsp. kosher salt
1 28- ounce can San Marzano whole peeled tomatoes puréed in a food processor or blender
1/4 tsp. crushed red pepper flakes
5 garlic cloves minced
1 medium yellow onion chopped
¼ cup olive oil