Heat the olive oil in a large saucepan. Add the scallions, bacon and rosemary and cook over moderate heat, stirring, for 5 minutes. Stir in the rice until coated with the oil. Add the wine and boil until almost evaporated, about 2 minutes. Pour in the mushroom soaking liquid, stopping when you reach the grit at the bottom of the measuring cup. Add 1 cup of the stock and cook over moderate heat, stirring constantly, until absorbed. Continue adding stock, 1 cup at a time, stirring until absorbed. The risotto is done when it is creamy and the rice is al dente, about 25 minutes total cooking time. Stir in the mascarpone and the porcini. Season the risotto with salt and pepper.