Place steak in a shallow dish; set aside. Combine chopped onion and next 6 ingredients in a blender; process until smooth. Reserve 1/2 cup ale mixture, and refrigerate. Pour remaining ale mixture over steak; turn to coat. Cover and marinate in refrigerator at least 8 hours, turning occasionally.
Remove steak from marinade; discard marinade. Place steak on grill rack coated with cooking spray; grill, covered, 5 to 6 minutes on each side or to desired degree of doneness, basting frequently with reserved 1/2 cup ale mixture. Garnish with thyme, if desired.