Prepare a hot fire in a charcoal grill or heat a gas grill or broiler. Away from the heat source, lightly coat the grill rack or broiler pan with cooking spray. Position the cooking rack 4-6 inches from the heat source.
Grill or broil the steaks until slightly pink on the inside, about 10 minutes each side. A food thermometer will read 145 for medium rare. Transfer to a plate and keep warm.
In a small saucepan, heat the margarine over medium heat. Add the garlic, mushrooms, thyme and rosemary. Saute lightly until the mushrooms are tender, about 1-2 minutes.
Remove from the heat and carefully add the whiskey (careful not to flame). Stir the sauce for another minute. Top the steaks with the mushroom sauce and serve immediately.