In a large skillet, heat oil over medium heat. Saute onion for about 3 minutes or until softened, add mushrooms, salt and pepper; saute for 5 - 7 minutes or until mushrooms are browned. If the pan gets too dry, add water as needed.
Add garlic, quinoa, rice and oregano; cook, stirring for 30 seconds or until grains are coated with oil and mixture is fragrant.
Add 2-2/3 cup stock and bring to a boil. Reduce heat to low, cover and simmer for 15 minutes or until liquid is absorbed and grains are tender. If liquid is absorbed before grains are ready, add more stock and continue to simmer, covered, until grains are tender.
Remove from heat and stir in parmesan, parsley and vinegar.