Put the 7 spices -- from the coriander seeds down to the garam masala -- in a mortar and pestle. Pound them together so the cardamom pods burst and the whole thing becomes a mess of spices. Discard the skin of cardamom pods. Continue to pound the spices until they become like a powder.
Heat the oil in a pot, add the onion and ginger over fairly high heat, and stir occasionally for a few minutes. Add the chili flakes and spices and stir for 2 minutes. You might need to add more oil (Spices slurp up oil as they fry)
Add the can of crushed tomatoes and stir. Cook until everything bubbles up together.
Add the chicken pieces, and stir to coat well, keeping heat high until everything is bubbling away. Turn the heat down, put on a lid, and simmer for about 20 minutes.
Add the garlic, sugar, lemon juice and salt to the curry. Cover with lid but leave a small opening for a steam to escape; simmer for 15 minutes or longer. Taste the sauce, and adjust seasoning according to your taste.
Near the end of cooking, skim off extra oil/fat with a spoon, as it collects in corners of the pot, if needed. Serve the curry warm with rice.