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Working over a fine-meshed stainless-steel strainer set over a bowl, scoop the seeds from the tomato wedges. Press the seeds to extract all the tomato juice. Stir the crème fraîche into the tomato juice and set aside.
In a medium saucepan, combine the tomatoes with the olive oil, rosemary, garlic and a pinch of salt. Cook over moderately low heat, without stirring, until the tomatoes are tender but still intact, about 10 minutes. Give the tomatoes a gentle push if necessary, to keep them submerged. Using a slotted spoon and allowing as much of the oil to drain back into the saucepan as possible, transfer the tomatoes to a plate. Reserve 3 tablespoons of the oil; strain and refrigerate the rest for later use.
Heat the chicken stock in a medium saucepan. In another medium saucepan, melt the butter in 1 tablespoon of the reserved tomato oil. Add the onion and cook over moderate heat, stirring occasionally, until softened but not browned, about 5 minutes. Add the rice and stir until coated with fat.
Add the wine and cook, stirring, until it is absorbed. Add the hot stock, 1 cup at a time, and cook, stirring constantly, until the rice is tender and most of the liquid is absorbed. Stir in the mascarpone, Parmesan and the remaining 2 tablespoons of the reserved tomato oil. Season with salt and pepper. Gently fold the olive oil-poached tomatoes into the risotto.
Put the tomato juice-crème fraîche mixture in a blender; blend at high speed until frothy. Spoon the risotto into 4 shallow soup plates, pour the tomato froth around it and serve right away.
The recipe can be prepared through Step 2 and refrigerated overnight.