- 6 tablespoons extra-virgin olive oil
- 5 cups diced yellow onions, red onions or shallots
- Fine sea salt
- 1 (28-ounce) can whole peeled, diced, crushed or puréed tomatoes
- 4 garlic cloves, thinly sliced
- 2 tablespoons tomato paste (optional)
- 4 to 5 basil stems (optional)
- 1/4 teaspoon red-pepper flakes (optional)
- Set a medium Dutch oven or similar pot over medium heat, and add 4 tablespoons olive oil. When the oil shimmers, add onions and a pinch of salt. Reduce heat to medium-low and cook, stirring regularly, until onions are lightly golden and tender, 16 to 18 minutes.
- In the meantime, pour the tomatoes into a large bowl and use hands to crush, if using whole tomatoes. Pour about 1.5 cups water into the can and swirl to rinse any remaining purée off the sides. Add the water to tomatoes in bowl and set aside.
- Once onions are soft and golden, add garlic and cook, stirring, until it threatens to turn golden, about 90 seconds. Add tomato paste, if using, and cook until color deepens, about 3 minutes. Add tomatoes and season with salt. If using, add basil stems and red-pepper flakes. Stirring regularly, allow sauce to come to a boil, then reduce heat to low, cover and simmer until sauce tastes savory and all raw tomato flavor is gone, about 45 minutes.
- Taste and adjust seasoning with salt and add remaining 2 tablespoons oil. Use a hand blender to purée, pass through a food mill or let cool, then purée using a blender or food processor. Tomato sauce can be made 1 day in advance and refrigerated or frozen up to 3 months.