Soak the chiles in boiling water until softened, about 10 minutes. Drain and puree with the marjoram, oregano, garlic, one-quarter of the chopped onion and the pineapple juice in a blender until smooth. Add 1 1/2 teaspoons salt and 1/4 teaspoon pepper. Blend to combine.
Combine the pork and the marinade with a fork in a large bowl. Cover with plastic wrap and refrigerate for 30 minutes.
Preheat the broiler. Divide the pork mixture into 6 equal portions. Shape each portion into a 3/4-inch-thick patty. Lightly oil a rimmed baking sheet; place the patties on the baking sheet and broil until slightly charred and cooked through, about 9 minutes per side. Transfer the burgers to a plate and cover with foil to keep warm. Let rest about 8 minutes.
Meanwhile, lightly oil another rimmed baking sheet, add the pineapple and broil until slightly charred, about 2 minutes per side.
Spread about 1 teaspoon mayonnaise on the cut sides of each bun and broil them until the mayonnaise has melted, about 2 minutes. Place the burgers on the bottom halves of the buns. Top with the pineapple, cilantro, the remaining chopped onion and the bun tops.
Photograph by Ryan Dausch
Courtesy of Food Network Magazine