Step 1In a large bowl, gently combine the beef chuck with the chopped onion, cilantro, Worcestershire sauce, jalapeno, salt, ground pepper and cayenne pepper. Form the mixture into 8 (half-inch to three-fourths-inch-thick) patties. Place the patties on a baking sheet. Cover and refrigerate at least 2 hours, up to 1 day.
Step 1In the bowl of a food processor, combine the habanero peppers, garlic, tomatoes, cilantro and vinegar. Process to a smooth paste. Taste and season as desired. This makes a generous 1 1/2 cups relish. Cover and refrigerate for 24 hours to give the flavors time to marry.
Step 1In a food processor, pulse together the mayonnaise, chipotle peppers, adobo sauce and lime juice. Season to taste with salt and pepper, then cover and refrigerate until ready to use. This makes about 1 cup chipotle mayonnaise, which will keep, covered and refrigerated, up to 1 week.
Step 1In a large skillet, preferably cast iron, melt the butter and olive oil over medium-high heat, then stir in the onions. Stir in the salt and pepper and continue to cook, stirring constantly, until the onions begin to soften, about 5 minutes. Stir in the sugar and continue to cook, stirring and scraping the browned bits from the bottom of the pan, until the onions are golden-brown, 20 to 30 minutes.
Step 1Working in batches, fry the jalapeno bacon slices in a large skillet over medium-high heat until browned and crisp. Transfer the bacon to paper towels to drain.
Step 2In another skillet, add the oil and fry the serrano chiles until softened and browned. Be careful, because the oil will splatter as the chiles fry. Remove the chiles and drain. Peel and seed if desired, and slice the chiles into strips.
Step 3Prepare the barbecue or grill and heat over medium-high heat. Spray the grill rack with nonstick spray. Toast buns until golden, about 2 minutes per side. Transfer buns, cut side up, to plates. Spread each bun bottom with some chipotle mayonnaise and top each with a lettuce leaf and sliced tomato.
Step 4Place the burgers on the grill and cook for 5 minutes. Flip the burgers and continue to cook to desired doneness, about 5 minutes longer for medium. Shortly before the burgers are done, top each burger with caramelized onion, the fried and sliced serrano chiles and a spoonful of habanero relish. Place a slice of habanero jack cheese on each burger and top with 2 slices jalapeno bacon. Continue cooking just until the cheese melts.
Step 5Place the burger patties on the bun bottoms, spread each bun top with a little more chipotle mayo and place over the burger. Serve immediately.