Heat olive oil in a large saucepan or pot over medium heat. Add onion and cook until softened, about 5 minutes. Add garlic and cook for 1 minute more.
Stir in rinsed barley and cook for 1 minute, stirring constantly, to lightly toast the grains.
Add about 1 cup of warmed vegetable broth to the barley, stirring constantly until almost all liquid is absorbed.
Continue adding broth, 1/2 cup at a time, stirring frequently and allowing each addition to be absorbed before adding more. This process will take about 25-30 minutes, until the barley is tender but still has a slight chew (al dente).
In a separate small skillet, sauté the sliced mushrooms over medium-high heat until browned and tender, about 5-7 minutes.
Once barley is cooked, stir in the sautéed mushrooms and Parmesan cheese (if using).
Season with salt and black pepper to taste. Stir in fresh parsley and serve immediately.
