Slice onion and sauté in 1tbsp of oil until browned on a medium heat, will take ~15-20mins, then transfer to a bowl
Add remaining 2tbsp of oil and temper the cardamoms, cinnamon, cloves and cashews for 30s until fragrant
Add in your browned onions, garlic, ginger, chilli and sauté
Add the tin of tomatoes followed by the ground spices, jaggery, and the kasuri methi and allow to simmer on low heat for 10 mins partially covered
Let this cool, remove the whole spices (I only bother with the black cardamom and cinnamon) and blend till completely smooth. Add water to help the blender if required
IMPORTANT - sieve your sauce for a smooth finish, and simmer until desired consistency.