2 1/2 cups light Mexican beer, such as Dos Equis
1/4 cup freshly squeezed lime juice (from 2 medium limes)
1/4 cup freshly squeezed lemon juice (from 1 medium lemon)
4 medium garlic cloves, crushed
1 medium red onion, coarsely chopped
1 teaspoon freshly ground black pepper, plus more as needed
2 to 3 pounds flank steak
2 tablespoons packed dark brown sugar
Combine 2 cups of the beer, the lime juice, lemon juice, garlic, onion, and measured pepper in a large, nonreactive bowl and mix. Add the flank steak, cover, place in the refrigerator, and marinate at least 30 minutes or overnight. When ready to cook the meat, heat a grill to high (about 500°F). Remove the meat from the marinade (reserving the marinade juices), pat it dry, and season well with salt.
Transfer the marinade to a medium saucepan, add the remaining 1/2 cup beer, and place over medium-high heat. Bring to a boil, skim off any foam or impurities that come to the surface, and let the mixture reduce by half, about 20 minutes. Strain through a fine-mesh strainer into a heatproof bowl, mix in the brown sugar, and season well with salt and pepper.
Place the 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. If serving as a main, slice the steak against the grain into 1-inch strips; if serving as a filling for burritos, coarsely chop. Place in a serving dish and cover with the reduced marinade juices.