Combine the boiling water and dried porcini mushrooms in a bowl. Allow to stand for 30 minutes, or until softened. Drain the mushrooms through a fine colander set over a bowl, reserving 1½ cups of the soaking liquid. Finely chop the rehydrated mushrooms and set aside.

In a small saucepan, combine the reserved mushroom soaking liquid with the beef broth. Bring to a gentle simmer, then reduce heat and keep warm. Do not allow the mixture to boil.

Heat a large saucepan over medium-high heat and coat lightly with cooking spray. Add the Arborio rice, chopped shallots, and minced garlic. Sauté for approximately 5 minutes, stirring frequently, until the rice is lightly toasted and the shallots are softened.

Add the white wine to the pan and cook for approximately 2 minutes, or until the liquid has nearly evaporated.

Add 1 cup of the warm broth mixture to the rice. Cook over medium heat, stirring occasionally, until the liquid is nearly absorbed. Continue adding the remaining broth mixture in ½-cup increments, allowing each addition to be absorbed before adding the next. This process will take approximately 25 minutes total.

Stir in the chopped porcini mushrooms, Parmigiano-Reggiano cheese, thyme, salt, and black pepper. Mix gently until the cheese has melted and the risotto is creamy.

Spoon the risotto into serving bowls and top each portion with approximately 1 tablespoon of mascarpone cheese. Serve immediately while warm.