Drain the chiles and put them in a blender with the marjoram, oregano, garlic, one-fourth of the chopped onion, and the pineapple juice, and puree until smooth. Season the marinade generously with salt and pepper.
Put the pork in large bowl and pour the marinade on top. Using a wooden spoon, mix well. Cover with plastic wrap and refrigerate for 30 minutes.
Heat a grill pan over medium-high heat.
Divide the pork mixture into 8 equal portions. Shape each portion into a 1/2-inch-thick patty. Oil the grill pan. Working in batches, grill the burgers until cooked through, 6 to 8 minutes per side. Transfer the burgers to a plate and cover with aluminum foil to keep warm. Let rest for about 8 minutes.
Meanwhile, brush the grill pan with a little more oil and grill the pineapple slices until golden brown, 2 minutes per side.
Spread mayonnaise on the cut sides of each bun and warm on the grill pan for 2 minutes.
Top the bottom of each bun with a burger, a slice of grilled pineapple, some cilantro, and some of the remaining chopped onion. Cover with the bun tops and serve.
Reprinted with permission from Mexican Made Easy by Marcela Valladolid, Clarkson Potter copyright (c) 2011