Combine onion, garlic, beer, ketchup, vinegar, mustard, chili powder, salt and pepper in large sealable freezer bag. Pierce steak all over with a fork; add to bag, turning to coat; seal and refrigerate for 8 to 12 hours.

Remove meat from marinade and reserve marinade. Pat steak dry with paper towel.

Grill steak in a closed preheated barbecue over medium-high heat (400°F/200°C) for 10 to 12 minutes for 1 inch (2.5 cm) thick steaks, turning twice or more, or until a digital instant-read thermometer inserted sideways into centre of steak reads 145°F (63°C) for medium-rare doneness at the least. Transfer to cutting board and let rest for 5 minutes.

Meanwhile, bring marinade to boil over high heat in a small saucepan; boil for 5 minutes or until slightly thickened.

Slice steak thinly across the grain on a 45-degree angle and serve with sauce.