2 1/2 cups light Mexican beer (such as Dos Equis)
1/4 cup freshly squeezed lime juice
1/4 cup freshly squeezed lemon juice
4 medium garlic cloves, crushed
1 medium red onion, coarsely chopped
1 teaspoon freshly ground black pepper
2 pounds flank steak
2 tablespoons dark brown sugar
Combine 2 cups of the beer, lime juice, lemon juice, garlic, onion, and pepper in a large bowl and mix. Add flank steak, cover well, place in the refrigerator, and marinate at least 3 hours or overnight.
Heat a grill to high or 500°F. Remove meat from marinade (reserving marinade juices), pat dry, and season well with salt.
Transfer marinade to a medium pot, add remaining 1/2 cup of beer, and place over medium-high heat. Bring mixture to a boil, skim off any foam or impurities that come to the surface, and let mixture reduce by half, about 20 minutes. Strain sauce through a fine mesh sieve, mix in brown sugar, and season well with salt and freshly ground black pepper.
Place meat on the grill and cook to medium, about 7 to 10 minutes per side. Remove from the grill and let rest for 5 to 10 minutes. Slice steak across the grain into 1-inch strips. Place in a serving dish, and cover with reduced marinade juices.