Heat a large skillet to high heat and add the oil. Wait 1 or 2 minutes for it to come up to the smoking point. Add the onion and garlic and toss furiously so that the garlic doesn't burn. When the onions become translucent add the mushrooms and toss until all mushrooms are golden brown and caramelized. Remove from the heat. Add butter and thyme and season with salt and pepper, to taste. Serve immediately.
Cook's Note: Never run mushrooms directly under water; they will absorb water like a sponge, and your saute will be far too soggy. Instead, using a slightly damp paper towel, wipe mushrooms down. Alternatively, use pastry brush and dust the dirt from the mushrooms.