Combine:
* 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
in a large bowl and mix to make marinade. Place marinade in a zip-top plastic bag and add 2 pounds of flank steak, zip to close and shake bag well. Refrigerate at least 4 hours.

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 bag, reserving marinade, pat it dry, and add salt to taste.

Transfer marinade to a medium pot, add 1/2 cup of beer, and place over medium-high heat. Bring mixture to a boil, skim off any foam, and let mixture reduce by half, about 20 minutes. Strain sauce through a fine mesh sieve, mix in 2 T. 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.