Preheat oven to 350 degrees.
Trim and quarter the onions. Thinly slice ¾ of one onion for pickling.
Combine water, sugar and salt in a microwavable dish. Microwave on high for 1 minute. Stir until the salt and sugar dissolve. Add the sliced onions to the mixture and pour the vinegar on top. Let sit uncovered until ready to serve.
Place the remaining onion, peppers, garlic and ginger in a food processor or blender and grind until it becomes a fairly smooth puree.
Heat oil in the bottom of an oven safe pot or large pan over medium heat. Add cumin seeds and allow them to brown a little and become fragrant. About 2-3 minutes.
Add the onion mixture to the pan and allow to cook until the liquid evaporates and it begins to brown. Be careful not to let it burn! **No need to clean the blender/processor yet, you’ll use it in the next step.
While the onion mixture is cooking, blend the tomatoes, salt, chili powder and garam masala in the food processor/blender.
Add the tomato mixture and the potatoes to the onion mixture and allow it to heat through on a medium heat. Cover and let cook on a medium low heat for 15 minutes.
Add the chicken to the mixture. Stir to combine. Drizzle with heavy cream and place in the oven, uncovered for approximately 45 minutes (check a potato for doneness).
Remove from the oven, scatter most or all of the pickled onion and cilantro on top. Serve with rice and enjoy!