Next, slice the mushrooms and put them in a roomy skillet. Coat them in oil. Sauté the mushrooms over high heat. They quickly absorb any oil, but eventually release it along with their juices, then brown. When golden, sprinkle with thyme, salt, and garlic, if using, and toss, or roast the mushrooms by preheating the oven to 400˚F. Put the sliced mushrooms in a wide, shallow baking dish or tray, toss with the oil, and season with salt and thyme. Bake until sizzling, about 25 minutes, then toss with the garlic.