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1. In a large saucepan, bring 8 cups of water to a boil. Whisk in the polenta in a slow, steady stream. Add 1 tablespoon of salt and cook over moderately low heat, whisking frequently, until the polenta is thickened and tender, about 30 minutes. Whisk in 4 tablespoons of the butter and season with salt and pepper. Press plastic wrap directly on the surface of the polenta to prevent a film from forming; cover and keep warm.
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2. Meanwhile, on a work surface, mince the parsley and garlic with the lemon zest. Transfer the gremolata to a small bowl.
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3. In a large skillet, melt 1 tablespoon of the butter in 2 tablespoons of the olive oil. Add half of the mushrooms and season with salt and pepper. Cook over moderately high heat, stirring occasionally, until the mushrooms are golden and tender, about 5 minutes. Stir in half of the chopped greens and cook for 1 minute; transfer the mixture to a large bowl. Repeat with the remaining butter, oil, mushrooms and chopped greens.
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4. Pour the polenta onto a large platter; if the polenta is very thick, reheat with 1/2 cup of water before serving. Spoon the mushrooms and greens on top and garnish with the mascarpone. Sprinkle the gremolata over the polenta and serve warm.
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MAKE AHEAD The cooked polenta can be refrigerated overnight. Rewarm with a little water before serving.
