Prepare Garlic Mayo: Combine mayonnaise, minced garlic, and lemon juice in a small bowl. Cover and set aside for at least 20 minutes to let the flavors meld.
Season and Prepare Chicken: Mix paprika, garlic powder, onion powder, oregano, salt, and pepper in a shallow dish If needed, pound chicken breasts to an even thickness of about 1 cm trim chicken slightly larger than bun size (it will shrink when cooked).
Cook Onions: Heat 1 tablespoon of olive oil in a skillet over high heat. Add sliced onion and sauté for 5 minutes or until golden brown season with salt and pepper, then set aside.
Toast Buns: Preheat the oven to 160°C (320°F). Toast burger buns until lightly crispy and leave them in the oven (turned off) to stay warm.
Cook Chicken: Heat the remaining tablespoon of olive oil in the same skillet over high heat. Dredge chicken in the seasoning mixture, shaking off excess, and place in the skillet. Cook for 2 1/2 minutes on one side until golden. Flip, top with cheese (if using), and cook for another 1 1/2 minutes. Cover briefly if the cheese hasn’t melted. Transfer chicken to a plate and cover loosely with foil to rest.
Assemble the Burgers: Spread mashed avocado on the bottom half of each bun and sprinkle with salt and pepper. Layer with cooked chicken, caramelized onions, lettuce, and tomato slices. Spread garlic mayo on the top half of the bun and place it on top.
Serve: Serve immediately with a side of fries, salad, or your favorite sides.