Place onions and garlic into a large saucepan and add water to cover. Bring to the boil and then reduce heat to medium-low. Cover and simmer for 45 minutes. Add the ginger and continue simmering for 5 minutes.
Puree the onion mixture in a food processor or blender, in batches, until smooth. Place a large saucepan over medium-high heat and pour in the diced tomatoes and spices. Bring to the boil, then pour in the onion puree. Reduce heat and simmer for 45 minutes.
I have this recipe in my slow cooker at the moment with some chicken pieces, and i tell you its worth making just for the smell alone! Its easy, smells divine and is a family favourite in my house. - 28 Jul 2011