1. Combine 2 cups boiling water and mushrooms; let stand 30 minutes or until soft. Drain through a colander over a bowl. Reserve 1 1/2 cups soaking liquid; chop mushrooms.
2. Bring soaking liquid and broth to a simmer in a small saucepan (do not boil). Keep broth mixture warm over low heat.
3. Heat a large saucepan over medium-high heat. Coat pan with cooking spray. Add rice, shallots, and garlic; sauté 5 minutes. Add wine, and cook until liquid evaporates (about 2 minutes).
4. Add 1 cup broth mixture to rice mixture; cook over medium heat 5 minutes or until the liquid is nearly absorbed, stirring constantly. Add remaining broth mixture, 1/2 cup at a time, stirring frequently until each portion of broth mixture is absorbed before adding the next (about 25 minutes total). Add mushrooms, Parmigiano-Reggiano, chopped thyme, salt, and pepper; stir gently just until the cheese melts. Spoon 1 cup risotto into each of 4 bowls. Top each serving with 1 tablespoon mascarpone and thyme leaves, if desired.