In a small saucepan, bring the cream with the vegetable stock, thyme and rosemary to the boil. Then remove the herbs and season the broth with salt and pepper.
Stir the polenta into the hot broth and cook on low heat for about 15 minutes, stirring occasionally.
In the meantime, clean and quarter the mushrooms. Cut the onions into fine strips.
Sear the onions and mushrooms in a little vegetable oil. Then sprinkle with sugar and deglaze with balsamic vinegar. Season to taste with salt and pepper. Let everything boil down for about one to two minutes. Serve immediately warm with the polenta.