Use this tomato sauce to make Eggplant Parmigiana and Rigatoni alla Norma from chefs Fabrizio and Nicola Carro.
Heat olive oil in a large pot over medium heat. Add onions and cook, stirring, until soft and translucent, 10 to 15 minutes.
Add tomatoes and reduce heat to low. Cook, stirring occasionally and taking care not to let sauce stick to the bottom of the pot, about 2 1/2 hours.
Remove from heat and, using an immersion blender, blend until smooth, adding basil leaves as you blend. Alternatively, working in batches, you can transfer tomato sauce to the jar of a conventional blender, adding basil leaves as you blend.
Season with salt and pepper and reheat to use immediately or let cool completely before transferring to an airtight container. Refrigerate until ready to use, 3 to 5 days.