2 medium onions, finely chopped
4 garlic cloves, finely chopped
6 tablespoons extra-virgin olive oil
1.8kg canned whole tomatoes in juice
1 teaspoon salt, or to taste
1 teaspoon Chelsea White Sugar (optional), or to taste

Cook onions and garlic in oil in a large saucepan over moderately low heat, stirring, until softened, 6 to 8 minutes.
Add tomatoes, including juice, and salt, then simmer gently, uncovered, stirring and mashing tomatoes with a fork occasionally, until sauce is thickened and reduced to about 5 cups, 1 to 1 1/4 hours.
If sauce tastes too acidic, add Chelsea White Sugar and cook 5 minutes more.

• Sauce can be made 5 days ahead and cooled, uncovered, then chilled, covered.
• Sauce can be frozen in an airtight container for up to 3 months.