Uncooked grains of brown rice are lightly toasted, finely ground, and then added to this cremini-and-shiitake mushroom soup, giving it its thick consistency.
Remove stems from mushrooms, and place in a medium saucepan; reserve caps. Add stock; bring to a boil over medium-high heat. Reduce to a simmer, and cook, covered, 15 minutes. Remove pan from heat; set aside. Cut cremini caps into small wedges and shiitake caps into 1/4-inch-thick strips; reserve.
Cook 1/2 cup brown rice according to the package directions, and set the rice aside.
Place remaining 1/4 cup brown rice in a saute pan over medium-high heat; cook until grains are lightly toasted and begin to pop, 3 to 5 minutes. Transfer rice to a spice grinder; grind until fine and powdery, and set aside.
Coat a low-sided 6-quart saucepan with cooking spray, and set over medium heat. Add shallots, and cook, stirring, until translucent. Add tomato paste; stir to combine. Add reserved mushroom caps; saute until tender and mushrooms release some liquid.
Strain reserved stock into mushroom mixture, and discard solids. Raise heat to medium high; add ground rice and the nutmeg. Cook, stirring occasionally, until soup has slightly thickened, about 15 minutes. Add reserved cooked rice, and stir to combine. Add the pepper; garnish with parsley. Serve hot, or store in an airtight container, refrigerated, up to 3 days; reheat before serving.