Add onion, carrots, and garlic to same pot, season with salt, and cook over medium-high, stirring occasionally, until onion is brown around the edges, 6–8 minutes. Add potatoes, Relajo, and red pepper flakes and cook, stirring constantly, 1 minute. Stir in vinegar and cook, stirring, until mostly evaporated, about 3 minutes. Add prunes, pineapple juice, and brown sugar. Bring to a simmer and cook, stirring occasionally, until liquid is reduced by half, about 5 minutes. Season with salt and add broth. Bring to a simmer and cook, stirring occasionally, until reduced by about one fourth, about 5 minutes. Nestle chicken back into pot, arranging skin side up. Cover and transfer to oven. Braise until meat is tender but not falling off the bone, 45–50 minutes.