In a medium bowl, mash ripe avocado with chopped cilantro, diced onion, garlic powder, lime juice, kosher salt, and black pepper until smooth and creamy, about 2-3 minutes. Taste and adjust seasoning as needed. Set aside.

In a large mixing bowl, combine ground chicken with fresh lime juice, chopped cilantro, chili powder, ground cumin, dried oregano, garlic powder, kosher salt, and black pepper. Mix gently until just combined, then form the mixture into four evenly-sized patties, about ½ inch thick each.

Heat 1 tablespoon of olive oil in a large skillet over medium-high heat. Once the oil is shimmering, add the sliced red bell pepper and yellow onion, sprinkling them with a pinch of salt and chili powder. Sauté for 6-8 minutes, stirring occasionally, until the vegetables are soft and slightly caramelized. Remove from heat and set aside.

In the same skillet, add another tablespoon of olive oil as needed, then carefully place the chicken patties in the pan. Cook for 4-5 minutes on one side until golden brown and cooked halfway through. Flip the patties, place a slice of Monterey Jack cheese on top of each, and continue cooking until the internal temperature reaches 165°F, about another 4-5 minutes.

If desired, toast the buns on the skillet or in a toaster for a few minutes until golden. Place the cooked Chicken Fajita Burgers on the bottom halves of the buns, top with sautéed peppers and onions, and a dollop of guacamole. Complete with the top bun.