Heat a grill pan or large skillet over medium high heat.
For Beef Fajita Burgers:
Combine sirloin, Worcestershire, spices, thyme, hot sauce and grill seasoning. Mark meat into 4 sections and make 4 patties, 1-inch thick. Using the reserved 1/4 onion piece, grate half of it on the burgers. Drizzle extra-virgin olive oil on the patties. Cook patties 4 minutes on each side for medium or, until desired doneness.
For Chicken Fajita Burgers:
Combine chicken, chipotle powder, cilantro, hot sauce and grill seasoning. Score and divide meat into 4 sections and form big patties, 1-inch thick. Using the rest of the 1/4 reserved onion piece, grate the other half of it on the burgers. Drizzle patties with extra-virgin olive oil and cook 6 minutes on each side or until meat is firm and cooked through.
Heat a medium skillet over high heat. Add extra-virgin olive oil and peppers and onions. Stir-fry the veggies tossing them with tongs to sear them at edges. Add the jalapeno or serrano pepper. Toss and turn the mixture about 3 minutes, then add salsa of choice and toss a minute longer. Place burgers on each bun bottom and top with 1/4 of the pepper and onion mixture and bun top. Serve Bacon and Bean Smash on the side.