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 to 6 inches from the heat source.
Grill or broil the steaks until slightly pink on the inside, about 10 minutes each side, or until a food thermometer indicates 145 F (medium rare), 160 F (medium) or 170 F (well-done).
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 to 2 minutes. Remove from the heat and carefully add the whiskey (be careful not to let the alcohol flame). Stir for another minute. Top the steaks with the mushrooms mixture and serve immediately.