Melt the butter in a large saucepan over medium heat. Add the diced onions & let them caramelize for a few minutes until a lovely golden brown & fragrant. (Be careful that the butter doesn't burn.)
Toss the steak with the flour & garlic pepper in a ziplock bag until all the steak pieces are coated evenly. Add them to the pan & stir to combine with the onions.
Pour the chicken stock in the saucepan. Continue stirring for a couple minutes as the sauce starts to thicken.
Add the mushrooms, Greek yogurt & Worcestershire sauce. Cover & simmer on low for 10-15 minutes.
Sprinkle with fresh thyme or parsley. Serve over pasta or rice with green veggies on the side. So delish!