Heat butter in a pot, stir in onions, cook for 4 - 5 minutes. Add carrots, mushroom, and rice. Stir another 5 minutes.
Add vegetable stock ( can also used chicken stock or water), thyme, and bay leaves.
First bring to a boil, then put in an oven dish, cover, and cook in a preheated oven at 450F for 20-30 minutes, until rice soaks the water.
Let it sit for at least 10 minutes before serving.