In a dutch oven or large pan cook the mushrooms, 1/2 tsp salt, and 1/4 tsp pepper in olive oil over medium heat. Stir often and cook until most of the liquid has evaporated, about 10 minutes. Remove mushrooms from the pan and set aside.
Melt the butter in the now empty pan and stir in the garlic and orzo, cook 2 minutes stirring constantly to prevent sticking. Stir in the vegetable broth and bring to a boil. Reduce heat to low and simmer, stirring occasionally, until liquid is absorbed, about 8 to 10 minutes.
Take off the heat and stir in the mushrooms and parmesan cheese, add salt and pepper to taste.