Pour the jamaica concentrate, bay leaves, whole cloves, black pepper and salt into a medium saucepan set over medium-high heat. Bring to a boil and simmer for about 15 minutes, or until it reduces to half the original amount.
Let the marinade cool and pour over the flank steak in a baking dish or container. Cover and let it marinade anywhere from 1 to 24 hours.
Set grill pan, grill or saute pan over medium-high heat. Brush with a light coat of oil. Sprinkle meat with a bit more salt.
Grill meat anywhere from 4 to 6 minutes per side, depending on how cooked you want it.
Meanwhile, pour the marinade into a saucepan set over medium heat and drop in the halved serrano or jalapeño chile. Let the sauce reduce for 15 minutes, or until it coats the back of a wooden spoon. The syrup will thicken considerably as it cools. Remove from heat and let cool.
When the meat is ready, slice it thin and if you want, cut it into chunks. Heat corn tortillas over a comal or skillet over medium heat, it will take about a minute per side. For each taco, add a couple tablespoons or slices of meat in the center of each tortillas, a slice of avocado, some crumbled queso fresco and drizzle some of the jamaica and syrup on top. Enjoy your tacos!