Heat butter and olive oil in a skillet over medium heat until butter melts.
Add in mushrooms. Cook until the mushrooms release their liquid and start to brown, about 10 minutes.
Add in sliced onions and Worcestershire. Cook until softened and caramelized about 15 minutes.
Add in garlic, salt, pepper and umami mushroom powder (if using). Continue cooking 1 - 2 more minutes or until garlic is fragrant. Adjust seasoning with salt and pepper if needed. Serve immediately.
Notes
Nutrition information for estimation purposes only.