Preheat oven to 350° and put a 2 1/4- to 2 1/2-qt. soufflé dish in oven to heat.
Cook 1 cup sugar, 1/4 cup water, and the cream of tartar in a large frying pan over medium-high heat, stirring occasionally, until sugar turns deep golden, 8 to 12 minutes. Remove from heat and stir in zest of 1 orange. Protecting hands, remove dish from oven and pour caramelized sugar inside. Tilt dish until syrup thickens and evenly coats bottom and 1 to 2 in. up side.
Heat a kettle of water to boiling. In a bowl, whisk remaining 2/3 cup sugar and zest of 1 orange with remaining ingredients. Pour into soufflé dish and set dish in a 9-in. square baking pan. Put pan with dish on oven rack. Pour boiling water into pan halfway up side of soufflé dish.
Bake until a knife inserted halfway between center and edge comes out clean, 1 1/4 to 1 1/2 hours. Remove dish from water and let cool on a rack, then chill, covered, at least 4 hours.
Run a thin metal spatula around inside of dish, then invert onto a deep plate. Set soufflé dish in hot water 10 minutes to loosen syrup (not all will come off); scrape syrup over flan. Cut flan into wedges.
I made this as an interesting alternative to the traditional pumpkin pie at Thanksgiving. All my guests loved it. The texture was creamy and smooth and melted in your mouth. I'm not sure what happened with the results an earlier review were posted. It was as creamy as any flan I've ever had. I changed absolutely nothing in the recipe and it turned out perfectly. I made it the day before I was serving it and it spent overnight in the refrigerator. I would do nothing different the next time.
This is an easy recipe to put together. I used canned pumpkin, which worked fine. The only drawback - and it's a minor one - is that the pumpkin makes this flan less smooth in texture than a flan normally is. However, the taste is great. I would make it again-
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