Stove top: In a large pot over medium heat melt the butter/oil. Add the onions and salt, and cook, stirring occasionally, until the onions soften - 10 minutes approximately.
Stir in the curry powder, coriander, cumin, and chile flakes, and cook just until the spices are fragrant and toasty - stirring constantly at this point about 30 seconds.
Stir in the tomatoes, the juices from the cans, and broth.
Simmer for fifteen minutes or so, then puree with a hand blender until smooth. If you'd like your soup even a bit thinner or with more of a coconut taste- you can thin it with more broth or more coconut milk. Taste and adjust with more salt if necessary.
Crock Pot: Follow directions #1 and #2 above. After sauteeing the onion and spices, pour that mixture plus the tomatoes and juices and broth into the crock pot. Simmer on HIGH for about 1 1/2 hours then LOW for about 3 hours longer.
About 5 minutes before serving, I poured in about 5 ounces of the coconut milk. Stir and taste. Add more coconut milk for a more pronounced taste, as desired.
Serving suggestions from the Apartment Therapy website: any or all of the following: cooked brown rice, lemon wedges, toasted almond slices, pan-fried paneer, fresh cilantro, parsley, thyme or oregano, a poached egg.
For the test recipe, we added to each serving of soup about 1/4 cup scoop short grain brown rice, a tablespoon or so (dollop) of thick coconut milk, fresh cilantro and lemon wedges.