Finely chop the sundried tomatoes and place them in a medium mixing bowl.
Add the ground chicken, garlic powder, dried oregano, sea salt, and black pepper to the bowl.
Gently mix the ingredients by hand until just combined, being careful not to overwork the meat.
Form the mixture into a thick, even patty approximately the size of your bun.
Heat the avocado oil in a non-stick skillet over medium heat.
Place the patty in the skillet and cook for 5-6 minutes per side, or until the internal temperature reaches 165°F.
While the burger cooks, lightly toast the whole-grain bun in a separate pan or toaster.
Place the cooked chicken patty on the bottom bun and sprinkle with crumbled feta cheese.
Top with the red onion slice and butter lettuce leaf, then close the burger with the top bun and serve immediately.