Preheat oven to 20 degrees Celsius.
Place sliced mushrooms on a baking paper sheet in a baking tray. Pour melted butter over the mushrooms and toss. Spring a little bit of salt over mushrooms and let them roast for 15 minutes.
Meanwhile, heat up olive oil in a large saucepan. Add onion and garlic. Sauteé for 2 minutes, then add cornstarch. Mix constantly until you don't see any lumps.
Add all the herbs and vegetable stock. Simmer over medium heat for 10 minutes. Then turn off the heat and transfer the soup to a food processor. Add 3/4 roasted mushrooms and process until smooth. Taste and add salt and/or pepper.
Divide soups in bowls. Serve with the rest of roasted mushrooms, fresh sage leaves, vegan sour cream, and fresh gluten-free bread rolls.
Enjoy!