Place tomatoes in a large saucepan and cook, covered, over medium heat for 10 minutes. Using a food mill placed over a large bowl, purée cooked tomatoes and juice, discarding skin and seeds.
Heat olive oil in a large saucepan, add onion and cook over medium heat, stirring for 8 minutes or until onion is translucent but not brown. Add purée and simmer, stirring occasionally, for 30 minutes or until the sauce thickens. Season to taste with sea salt and freshly ground black pepper. Passata will keep, refrigerated in an airtight container, for up to 1 week, or frozen for up to 3 months.