This rolled flank steak is stuffed with all the flavors of Puerto Rico. Sweet plantains, yellow rice and black beans bring the essence of the island, while the roasted garlic seasoning and barbecue sauce amp up the flavor.
For the rice: Heat 1 teaspoon of the oil in a small saucepan over medium heat. Add the turmeric and cook, stirring, until toasted, about 30 seconds. Add the rice and stir to coat. Add 1 cup water and 1/2 teaspoon salt. Bring to a boil, reduce to a low simmer, cover and cook until all of the water has been absorbed and the rice is tender, about 15 minutes. Remove from the heat and stir in the mayonnaise and garlic. Adjust the seasoning with salt and let cool completely. Set aside.
Prepare a grill for direct and indirect heat: For gas grills (with 3 or more burners), turn all the burners to medium-high heat; after about 15 minutes, turn off one of the side burners and turn the remaining burners down to medium. For charcoal grills, bank one chimney starter-full of lit and ashed-over charcoal briquettes to one side of the grill. Set up a drip pan on the other side to avoid flare-ups. (Be sure to consult the grill manufacturer's guide for best results.) If your grill has a thermometer, aim to keep the temperature at about 350 degrees F throughout grilling.
For the peppers and plantains: Heat 2 tablespoons of the oil in a medium flameproof skillet over the direct-heat side of the grill until hot and shimmering. Add the peppers and cook until softened, about 5 minutes; set aside. Add the plantain halves cut-side down and cook, flipping once, until golden and caramelized, about 5 minutes per side. Let cool, then cut one of the plantain pieces in half lengthwise to make 3 strips total. Set aside.
For the beans: Put the beans and 2 tablespoons of the barbecue sauce in a small bowl. Add a pinch of salt and squish the beans with your hands until broken down enough to hold together but not completely a paste. Set aside.
For the flank steak: Set the flank steak on a cutting board with a short end facing you. Starting at one long side, slice the steak in half horizontally, leaving 2 inches at the opposite long side uncut so the pieces stay together. Open up the steak like a book so you have 1 big piece. Pound the steak with a meat mallet to an even thickness of about 1/2 inch. Sprinkle both sides of the steak with the roasted garlic and herb seasoning.
Position the steak vertically in front of you so the short ends are at the top and bottom of the cutting board. Pat the rice into an even layer all over the steak. Starting a third of the way in from the left side of the steak, lay down one plantain strip vertically on the steak. Moving right, line up 4 scallions next to it, followed by a third of the peppers and a third of the beans. Repeat 2 more times with the remaining ingredients, starting with a plantain strip and ending with the beans. Starting at one of the long ends, roll the steak around the filling like a jelly roll. Tie closed at 1-inch intervals around the circumference with kitchen twine. Tie 1 more piece of twine lengthwise around the meat to secure it. Coat the meat evenly with the remaining 1 tablespoon oil.
Lightly oil the grill grates. Grill the steak on the direct-heat side of the grill to brown all 4 sides, about 10 minutes total. Move the steak to the indirect-heat side of the grill, cover the grill and cook, turning occasionally and basting with the remaining 1/4 cup barbecue sauce during the last 5 minutes of grilling; cook, testing the temperature frequently, until the very center registers 115 degrees F on an instant-read thermometer for a steak that is well done on the outside and medium rare in the center, about 30 minutes more. Transfer the steak to a cutting board to rest for 15 minutes.
Untie the steak and slice on a slight bias into 6 thick slices. Serve with more barbecue sauce on the side.