Heat gas or charcoal grill. With small metal spoon, scrape underside of mushroom caps to remove dark gills. Spray mushroom caps with cooking spray. Place mushroom caps, underside down, on gas grill over medium-high heat or on charcoal grill 4 to 6 inches from medium-high coals. Cook 8 to 10 minutes or until mushroom caps are tender, turning occasionally. Remove from grill; turn underside up.
Meanwhile, cook spinach as directed on box. Squeeze to drain well.
In 2-quart saucepan, heat oil. Add onion; cook and stir 1 minute. Stir in broth, curry powder and salt. Heat to boiling. Stir in rice; reduce heat to medium-low. Cover and cook 3 minutes. Stir in spinach and carrot; cover and cook 4 to 6 minutes longer or until rice and vegetables are tender. Remove from heat; let stand 3 to 4 minutes or until liquid is absorbed; fluff with fork.
Carefully spoon about 1/2 cup rice mixture onto each cap. Serve with yogurt and green onion.