In a medium bowl, combine the boiling water and dried porcini mushrooms. Allow them to soak for approximately 30 minutes, or until softened. Drain the mushrooms through a fine mesh sieve lined with a paper towel over a bowl, reserving 1 1/2 cups of the soaking liquid. Coarsely chop the rehydrated mushrooms and set aside.

In a small saucepan, combine the reserved mushroom soaking liquid with the beef broth. Place over low heat and maintain at a gentle simmer throughout the cooking process. Avoid boiling.

Coat a large saucepan with cooking spray (or heat 1 tablespoon of olive oil) over medium-high heat. Add the Arborio rice, shallots, and minced garlic to the pan. Sauté for approximately 5 minutes, stirring frequently, until the shallots are softened and the rice is lightly toasted.

Add the dry white wine to the pan, stirring constantly. Cook until the liquid is nearly evaporated, which will take about 2 minutes.

Add 1 cup of the warm broth mixture to the rice. Cook over medium heat, stirring occasionally, until most of the liquid has been absorbed. Continue adding the remaining broth mixture, 1/2 cup at a time, allowing each addition to be absorbed before adding the next. Stir frequently to ensure even cooking. This process will take approximately 25 minutes.

Once the rice is creamy and tender, stir in the chopped porcini mushrooms, grated Parmigiano-Reggiano, chopped thyme, salt, and freshly ground black pepper. Stir gently until the cheese has fully melted and the ingredients are well incorporated.

Spoon the risotto into serving bowls. Top each portion with a generous tablespoon of mascarpone cheese, allowing it to melt and blend into the risotto for a rich, creamy finish. Serve immediately while warm.