Preheat oven to 400 F.
In a large Dutch Oven add the butter and pop into the oven to melt. As soon as it's very hot, add the onions, flat side down. Sprinkle with salt. Let the onions cook for 5 minutes. Once all the onions have sizzled, pop into the oven and roast for 20 minutes.
Make the quick curry base by combining the garlic, peeled tomatoes, tomato paste, tamarind, and salt in a food processor. Add the curry and chili powders and blitz until a smooth paste.
Take the onions out of the oven and lower the heat to 350 F. Flip the onions over onto their rounded sides.
Pour the curry mixture in and around the onions, put the lid on, and place back in the oven and roast for 25 minutes.
Remove from the oven and take lid off. Drizzle with cream and sprinkle with cilantro.
Serve hot with steamed rice or naan.