Heat 1 tablespoon oil in a large pan and fry chopped onions on low heat for 10 minutes until soft but not brown.
Add garlic, ginger, and salt to the onions, then add just enough water to cover the mixture. Simmer uncovered for 30 minutes.
In a separate pan, heat the remaining 5 tablespoons of oil and add tomatoes, tomato puree, ketchup, turmeric, cinnamon, and paprika. Bring to a boil, then reduce heat and simmer for 10 minutes.
Blend both mixtures separately until very fine and smooth.
Combine both blended mixtures in one pan and simmer for 15 minutes.
Add water gradually to achieve desired consistency, aiming for approximately 1600ml of gravy.
Allow to cool completely before storing or using immediately for your curry dishes.