Place mushrooms in small dish, pour in ½ cup warm water, soak for 15 minutes. Heat broth till hot. In saute pan, place in olive oil and chopped onion, place in rice and toast for 30 seconds, then pour in Prosecco. When liquid is absorbed, pour in ¾ cup broth, stir, and when liquid is absorbed repeat ,place in mushrooms with their water and stir, then repeat with broth and stirring till al dente. Remove from heat. Top with butter and grated parmigiano cheese, stir and serve hot.