Heat vegetable oil over medium-high heat in a medium-large pot until shimmering. Add chopped onion, minced garlic, and thinly sliced carrots, sauté for about 4 minutes until fragrant and onions are translucent.

Add thinly sliced brown mushrooms to the pot and sauté for an additional 3 minutes until the mushrooms reduce in size and start to brown.

Stir in barley, ensuring it's mixed with the sautéed vegetables. Pour in water until just covered. Add vegetable stock powder, salt, and ground black pepper, then bring to a boil.

Reduce heat to low, cover, and simmer gently for about 1 hour, stirring occasionally until the barley is tender.

For a slow cooker, transfer the sautéed vegetables, add water, barley, and seasoning, then cook on low for 6-7 hours.

Ladle soup into bowls, garnish with freshly chopped parsley, and enjoy with crusty homemade bread.