Preheat the oven to 375°F.
In a medium skillet, heat 1 tablespoon olive oil over medium heat. Add onion and sauté until tender, 4-5 minutes. Add garlic and cook until fragrant, about 1 minute more.
Transfer onion and garlic to a large bowl and let cool slightly. Stir in the cilantro, 5-spice, cayenne, mustard, salt, pepper, soy sauce, rice vinegar, and egg. Mix well to combine.
Add the ground chicken and mix to combine. Add breadcrumbs and mix just until fully incorporated. Divide meat mixture into six even portions, and form into patties.
In a large oven-safe skillet, heat the remaining 3 tablespoons olive oil over medium heat. Add the patties to the pan and cook until evenly browned, 3-4 minutes per side. Glaze both sides of each patty with 1 tablespoon sweet chili sauce, and transfer the pan to the oven. Continue to cook until the patties are fully heated through, 8-10 minutes more.
Meanwhile, prepare the broccoli rabe: in another large pan, heat olive oil over medium heat. Add garlic and reduce heat to low. Cook for 1 minute, then add the broccoli rabe and continue to cook until tender, about 5-6 minutes. Season with salt and pepper. Keep warm.
Prepare lemon mayonnaise: in a small bowl, stir the mayonnaise, lemon zest, and lemon juice together until combined.
To serve, spread mayonnaise onto each bun, then place a patty on top, followed by the broccoli rabe. Finish with the top bun, and serve immediately.
Recipe adapted from: www.andyboy.com