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