In a medium mixing bowl, combine the ground chicken, egg white, sea salt, black pepper, and garlic powder.
Gently mix the ingredients by hand until just incorporated, being careful not to overwork the meat to ensure a juicy texture.
Form the chicken mixture into a single uniform patty, approximately 1/2 inch thick.
Heat the avocado oil in a non-stick skillet over medium heat until the oil shimmers.
Place the patty in the skillet and cook for 5 to 6 minutes per side, or until the internal temperature reaches 165°F and the exterior is golden brown.
While the patty is cooking, lightly toast the whole wheat bun in a toaster or on a separate dry pan until the edges are crisp.
Slice the avocado and prepare the lettuce, tomato, and onion slices.
Place the cooked chicken patty on the bottom half of the toasted bun.
Layer the avocado, lettuce, tomato, and red onion on top of the patty, then cover with the top bun and serve immediately.