1. In a small oven-proof pot heat the oil over medium-low heat. Add the onions and thyme and cook until the onions become translucent but not brown, about 3 to 4 minutes.
2. Add the barley, pinch of salt and a few twists of freshly ground pepper. Stir to coat the barley evenly with oil. Cook for 3 to 4 minutes.
3. Add the stock and the mushrooms and bring to a boil over high heat.
4. When the stock boils, cover the pot and place in the oven until the barley is tender, about 30 minutes.
5. Adjust the salt and pepper and fluff with a fork.