In a medium saucepan, bring 1 kilogram water to a boil. Add salt and sugar and stir until fully dissolved. Remove from heat and add all remaining ingredients. Let cool to room temperature.
Place lamb in a large container. Submerge in Brine. Cover and refrigerate 12 hours.
In a blender, purée all ingredients and 500 grams water for the Adobo Marinade. Set aside.
Remove Lamb from Brine. Transfer to a nonreactive container. Coat with Adobo Marinade. Cover and refrigerate 8 hours.
Heat oven to 465°F. Remove Lamb Barbacoa from Adobo Marinade, reserving excess Adobo Marinade separately.
In a cast iron skillet over medium-high flame, heat oil. Add Lamb Barbacoa and sear on all sides.
Transfer seared Lamb Barbacoa to a hotel pan. Add enough water to fill the pan halfway. Add reserved Adobo Marinade. Top with banana leaves. Cover pan with aluminum foil and roast 1 hour.
Reduce heat to 355°F and cook additional 90 minutes, or until meat is tender and begins to fall off the bone. Let cool.
In a shallow serving bowl, spoon black beans. Top with Lamb Barbacoa. Spoon excess Adobo Marinade onto the beans. Garnish with cilantro and white onion.
