Clean and cut the mushrooms into eighths. Peel and finely chop the onion and garlic and sauté in a saucepan with 1 tablespoon of butter. Steam the rice briefly. Deglaze with water and add the stock cube and vegetable stock. Let the rice simmer for about 15 minutes (if the packaging shows a significantly different preparation value, then please adjust), stirring constantly, simmer gently without the lid, then remove the pot from the stove. Stir in the remaining butter and the parmesan.
Heat the olive oil in a pan and steam the mushrooms in it for about 3-5 minutes - if necessary add a little water to prevent them from burning. Fold the mushrooms into the rice and season everything with salt and pepper.