Heat up your skillet on medium high heat and add olive oil and butter. Add your chopped onion and garlic. Adding them together keeps the garlic from getting too dark. Let cook for about a minute or so.
Add your cut mushrooms and the rest of the seasonings. Cook well turning often until mushrooms are cooked and excess water from them has reduced down to a nice sauce. Do not cover as the water in the mushrooms needs to evaporate and reduce as they cook. Serve. This tastes better the next day when all the flavours have steeped!