How to Make It
In a heatproof bowl, soak the dried porcini in the boiling water until softened, 10 minutes. Drain, reserving 1 cup of the soaking liquid; rinse the mushrooms to remove any grit. Finely chop the porcini and reserve in a small bowl. Pour the soaking liquid into a medium saucepan, stopping before you reach the sediment at the bottom. Stir in the chicken stock and season with salt and pepper. Warm the stock over low heat.
In a large saucepan, heat 2 tablespoons of the oil until shimmering. Add the shallot and garlic and cook over moderate heat, stirring, until softened, 2 minutes. Add the rice and dried porcini and stir to coat. Add the wine and bay leaf and cook until the wine has evaporated. Add about one-fourth of the warm stock and cook over moderate heat, stirring constantly, until nearly absorbed. Continue adding the stock in batches, stirring constantly until the rice is al dente and suspended in a creamy sauce, about 20 minutes. Discard the bay leaf. Stir in the butter, mascarpone and cheese; season with salt and pepper and keep warm.
In a skillet, heat the remaining 2 tablespoons of oil. Add the fresh porcini and thyme and cook over high heat, stirring, until softened and golden, 8 minutes. Discard the thyme. Season the porcini with salt and pepper, spoon over the risotto and serve.