Start by warming a large cooking pan over medium/low heat. Add about a tbsp of oil and lightly cook the onion until it's translucent. Now add the garlic and cook it for 1 more minute. Add cumin and mustard seeds and cook for another minute before adding mushrooms. Cook the mushrooms until they start releasing most of the moisture. Now add the celeriac, bouillon cubes and water Bring the pot to a boil and let it simmer over medium heat for 25 minutes. Add the miso paste and blend the soup until smooth. Add salt and pepper to taste. Optionally top with some herbs, seeds, roasted chickpeas or croutons and enjoy with some bread.