In a medium saucepan, heat the olive oil over medium heat. Add onion and saute for about 10 minutes, stirring frequently until soft and translucent but not browned.
Add the garlic and red pepper flakes and cook until fragrant, stirring often to ensure the garlic does not brown.
Add the jar of passata, water and sugar. Tip: add the water to the empty jar and swirl to get any remaining passata left in the jar.
Simmer sauce over low heat, stirring frequently while you prepare the remainder of the recipe.