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Preheat the oven to 400°.
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In a small saucepan, melt the honey and butter with the lemon juice over moderately low heat.
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Scrape the honey syrup into a medium bowl, add the halved plums and toss to coat thoroughly. Arrange the plums, cut side down, in a large baking dish and spoon half of the honey syrup evenly over them.
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Roast the plums for 15 minutes, or until barely tender and beginning to brown on the bottom. Turn the plums and spoon the remaining honey syrup over them.
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Roast the plums for 10 minutes longer, or until tender but not falling apart. Transfer the plums to plates and drizzle with the honey syrup.
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Top with a dollop of mascarpone, sprinkle with the pistachios and serve.