Clean mushrooms by using a paper towel dampened with water to gently remove dirt (or use a mushroom brush). Slice into ⅓ to ½ inch slices.
Heat olive oil and butter in a large skillet over medium heat.
Once butter has melted, add mushrooms, soy sauce, garlic powder, salt and pepper. Using a wooden spoon, gently stir to coat mushrooms as evenly as possible. Using the spoon, arrange in a single layer.
Allow the mushrooms to cook (without stirring) for approximately 5 to 6 minutes, allowing them to brown.
Stir to flip the mushrooms and cook for an additional 4 to 5 minutes (without stirring), or until well browned and tender.
Sprinkle with chopped fresh thyme or parsley when serving, if desired.