This curry base recipe will likely make slightly more curry base than you need. The rest can be frozen or used for another curry recipe. It can also be made a day in advance.
Finely chop three of the garlic cloves. Add about 2cm of oil in a frying pan and add the finely chopped garlic. Cook until golden brown, but watch and not burn it or it will taste bitter. Drain, retaining the oil, and keep the crispy garlic in a bowl to use later.
Add a tablespoon of the garlic flavoured oil into a large saute pan and heat over a medium heat. Add the sliced onion and cook gently for 20 minutes until deep brown and golden.
Blitz the remaining 4 garlic cloves and the ginger with a teaspoon of the garlic oil in a blender until you have a paste.
Add the paste, the cinnamon stick, cumin, coriander and chilli powder to the pan.
Bash the cardamom pods with a knife and add to the pan. Stir to combine and cook for a couple of minutes
Add the diced tomatoes, passata and top up with water to cover. Season with about a teaspoon of salt and simmer for 15-30 minutes until thick and saucy. If it looks too dry whilst cooking, add a little water.
Stir through the crispy garlic and check the seasoning. The curry base is now ready to use or can be cooled and refrigerated for up to two days.