How to Make It
Preheat the oven to 400°. In a small saucepan, melt the honey and butter with the lemon juice over moderately low heat. Stir in the orange flower water, if using. 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.
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. 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. Top with a dollop of mascarpone, sprinkle with the pistachios and serve.