Begin by heating 2 tablespoons of vegetable oil in a medium-large pot over medium-high heat until shimmering, about 1-2 minutes. Next, add 1 chopped onion, 2 minced garlic cloves, and 2 sliced carrots. Sauté the mixture for about 4 minutes, stirring occasionally, until the onion becomes soft and translucent, releasing its aromatic fragrance.

Once the base is fragrant, incorporate 8 ounces of thinly sliced brown mushrooms into the pot. Continue to sauté for an additional 3 minutes, allowing the mushrooms to brown slightly and absorb the flavors, creating a deep umami essence. Stir occasionally to ensure even cooking and prevent sticking.

Stir in 1 cup of pearl barley, mixing well with the sautéed vegetables. Then, pour in enough water to cover the ingredients by about 4 inches. Season the mixture with 1 tablespoon of vegetable stock powder, along with salt and pepper to taste. Increase the heat to bring the soup to a rolling boil.

Once boiling, reduce the heat to low and cover the pot with a lid. Let the Vegetarian Mushroom Barley Soup simmer for 1 hour, stirring occasionally.

For a slow cooker version, layer your sautéed vegetables at the bottom of the slow cooker. Add the barley, water, and seasonings, then stir everything together. Set the slow cooker on low heat and allow it to cook for 6-7 hours.

After the simmering is complete, check the barley for doneness, ensuring it's tender but still retains a slight chew. Serve the hearty Vegetarian Mushroom Barley Soup warm in bowls and garnish with finely chopped fresh parsley.