In a large, 3-quart saucepan or a sauté pan with a cover, melt the butter over medium heat until it stops foaming. Reduce the heat to low and add the onion. Cook, stirring occasionally, 1-2 minutes, until soft and transparent. Fold in the rice and stir to coat in butter. Raise heat to medium, and cook, stirring, for 2 minutes. Pour in the stock, and add the thyme sprig and bay leaf. Cover and simmer over low heat for 20 minutes, until the stock is absorbed and the rice is tender. If needed, add up to 1/3 cup more stock and cook for a few minutes longer.

Remove from heat and fluff with a fork. Remove the bay leaf and thyme sprig, and add salt and pepper if needed. Serve hot.