If mushrooms are greater than one inch in diameter, cut into quarters.
Heat 12 inch skillet over medium heat.
Add 1 T olive oil. When the oil is hot enough to sizzle a piece of mushroom, add half the mushrooms and half the thyme. Raise the heat to medium-high and saute, stirring only occasionally, until the mushrooms are golden brown. If you stir them too often or crowd them in the pan, you'll prevent the mushrooms from carmelizing.
Season with salt and pepper.
Transfer the mushrooms to a bowl, and repeat with the remaining mushrooms. Add the garlic, and saute 1 minute, stirring. Return the reserved mushroom, with any accumulated juices, to the pan.
Add the sherry and parsley, and boil until the liquid is reduced to a glaze, another minute or so.
Add the butter, and gently swirl the pan as the butter melts and emulsifies with the sauce. Taste and adjust the seasonings with salt and pepper.