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1
Arrange oven rack in middle position and preheat oven to 375u00b0F.
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2
Cut peel, including white pith, from 3 oranges with a small sharp knife. Working over a bowl, cut segments free from membranes. Squeeze juice from membranes into bowl. Drain off as much juice as possible from segments into a measuring cup. Squeeze enough juice from remaining orange to total 2/3 cup of juice.
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3
Heat 2 tablespoons oil with butter in a 12-inch heavy skillet over moderate heat until butter is melted, then cook onion, stirring occasionally, until softened and golden brown, 18 to 20 minutes.
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4
While onion is caramelizing, cook rhubarb, honey, 1/3 cup orange juice, and 1/4 teaspoon salt in a 2-quart heavy saucepan over moderate heat, stirring occasionally, until rhubarb is very tender, about 12 minutes. Force mixture through a medium-mesh sieve into a bowl, scraping and pressing hard on solids and discarding them.
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5
Add ginger and cinnamon to onion and cook, stirring, 1 minute. Add remaining 1/3 cup orange juice and boil until thickened, about 1 minute. Stir in rhubarb puree, tomatoes, pepper, and 1/2 teaspoon salt and briskly simmer, stirring occasionally until sauce is thickened slightly, 7 to 10 minutes. Remove from heat.
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6
While sauce simmers, grease a 17- by 12-inch glass or ceramic baking dish with remaining tablespoon oil. Pat fish dry and sprinkle with remaining 1/2 teaspoon salt. Arrange fish, skin sides up, in 1 layer in baking dish and bake until opaque and just cooked through, 12 to 14 minutes. Stir orange segments into sauce and cook until heated through, 1 to 2 minutes, and season with salt. Stir mint into sauce and serve with fish.
