Clean the mushrooms or rub them with a damp kitchen towel. Cut the mushrooms into small pieces except for a few.
Peel the onion and cut into small cubes. Rinse, dry and pluck the thyme.
Heat the oil in a stock pot, sauté the onion cubes while turning them. Add the mushrooms and 1 teaspoon thyme and stir-fry over a high heat for about 3 minutes. Remove about a third of the mushrooms with a slotted spoon and set aside.
Scatter oat flakes over the remaining mushrooms and sauté for about 1 minute while stirring. Add the water and cream and also the vegetable stock and stir thoroughly. Then let everything simmer over low heat for about 5-6 minutes, stirring occasionally.
Puree the soup finely and season well with salt and pepper. Arrange the remaining mushrooms and the remaining thyme on the cream soup.
Tip: You can simply use milk instead of cream and water. Or milk with cream, which then becomes very creamy.