Rinse 150g pearl barley in a colander with cold tap water until water runs clear. Let it drain.
Heat 2 tbsp olive oil in a large pot or Dutch oven over medium-high heat. Add 1 onion (finely diced), 2 carrots (chopped), 2 celery stems (chopped), 1 swede (chopped), 150g mushrooms (cut), and 2 garlic cloves (minced). Cook for 5 minutes, stirring regularly, until vegetables begin to soften.
Stir in 1/2 tsp thyme, 1/4 tsp ground coriander, and 1/4 tsp ground fennel. Cook for 1 minute more until fragrant.
Add the rinsed pearl barley and 1.5 litres vegetable stock to the pot. Stir everything together, bring to a boil, then reduce heat to low, cover, and simmer for 35 minutes, or until the barley is tender with a slight chew.
Stir in fresh parsley leaves just before serving.
Ladle the hearty soup into bowls and serve immediately.