Combine teriyaki sauce, 1-1/2 t. chopped garlic, vinegar, Worcestershire and pepper sauce in medium-size non-aluminum bowl. Add steak and turn several times to coat. Cover with plastic wrap. Refrigerate at least 30 minutes or overnight.
Preheat broiler or grill.
Prepare sauce: Stir together cornstarch and broth in glass liquid measure until combined. Heat margarine in small saucepan over medium heat until bubbly. Add onion and remaining chopped garlic; cook 2 minutes or until onion begins to soften. Remove from heat.
Carefully add whiskey and mustard; stir to combine. Return pan to heat and bring to boiling. Cook 3 to 4 minutes or until reduced by about half. Re-stir cornstarch mixture and add to saucepan along with parsley and pepper; stir to combine. Return to boiling. Reduce heat to low; simmer 4 to 5 minutes, stirring occasionally. Cover saucepan and keep warm while cooking beef.
For broiling: Place steak on broiler rack; discard marinade. For rare beef, broil 5 inches from heat for about 8 minutes, turning meat halfway through.
For grilling: Place meat on a grill over medium-high heat. Grill steak for 5-8 minutes on each side.
Let meat stand 5 minutes. Slice across grain. Serve with sauce.