Add the butter to a deep skillet along with the sliced onions. Sauté the onions over medium heat for a few minutes, or just until they begin to soften. Add the sliced mushrooms, a pinch of salt and pepper, the thyme, and oregano. Sauté the mushrooms until they release all their moisture and the liquid in the bottom of the skillet has dried up (about 10 minutes). You want to see some of the liquid browning and coating the bottom of the skillet (but not burning).