Heat the olive oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it starts to soften.
Stir in the garlic and mushrooms. Cook for 5 minutes until the mushrooms release their liquid and begin to brown.
Add the carrots, parsnips, and thyme. Cook for 2 minutes, stirring often.
Pour in the vegetable broth and add the rinsed barley, salt, and pepper. Bring the soup to a boil.
Reduce the heat to low, cover the pot, and simmer for 35 minutes until the barley is tender and the vegetables are soft.
Turn off the heat and stir in the fresh parsley before serving.