- Arrange a rack in the upper third of the oven and preheat to 350°F. Grease a 2-qt. baking dish (such as 8x8x2-inch) with butter. Place the dish on a foil-lined rimmed baking sheet.
- Heat cream, butter, garlic, salt, thyme and pepper in a small saucepan over medium heat just until simmering. Remove from heat.
- Arrange half the potatoes in an even layer in the baking dish. Sprinkle with ¼ cup of the mushrooms and half the cheese. Pour half the cream mixture over the cheese. Repeat layering with remaining potatoes, ½ cup mushrooms (¼ cup should remain) and remaining cheese and cream mixture. Top with remaining ¼ cup mushrooms.
- Bake about 1 hour and 15 minutes, uncovered, until potatoes are very tender and the top is dark golden brown (tent with aluminum foil if the top starts to get too brown). Let cool 10 minutes for sauce to thicken before serving.
Recipe can be doubled. Prepare as recipe directs, except bake in a greased 9×13-inch baking dish for 1 1/2 hours.