1. Combine olive oil, whiskey, garlic, salt, and thyme in a plastic bag. Add steak and refrigerate for 1-8 hours. Do not throw the marinade away.
2. In a small saucepan, heat the marinade to a full rolling boil. Keep at a full boil for 1 minute.
3. Add the onions and mushrooms to the cooked marinade. Cook over medium heat until the mushrooms have released their liquid and vegetables are tender and sauce has been thickened (15-20 minutes).
4. Preheat a grill to medium-high. Grill steaks to 145 degrees F (medium). Let rest 10 minutes. Top with sauce and vegetables to serve.