Prepare the Barley: Rinse the barley well in a fine-mesh strainer under cold water.
Place it in a saucepan, cover with water, and bring to a boil.
Cook for 2 minutes, then remove from the heat, cover, and let it stand for 1 hour.
Drain thoroughly.
Sauté the Aromatics: Preheat your oven to 350°F / 175°C.
In a medium flameproof baking dish or Dutch oven, melt the butter over medium heat.
Add the quartered mushrooms, minced onion, and dried thyme.
Sauté for about 10 minutes, stirring occasionally, until the onions are soft and the mushrooms have started to brown.
Combine and Bake: Remove the dish from the heat.
Stir in the drained barley, vegetable stock, tamari, salt, and a few grinds of fresh pepper.
Cover the dish tightly with a lid or foil and transfer to the preheated oven.
Bake for 1 hour, or until the barley is tender and all the liquid has been absorbed.
Rest and Serve: For the best texture, let the pilaf rest, covered, for 5-10 minutes after removing it from the oven.
Do not stir until just before serving. Fluff with a fork and serve immediately.
