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.