Combine teriyaki marinade, sweet chili sauce, olive oil, golden syrup, and grated ginger in a ceramic dish.

Add steaks to the marinade, ensuring they are fully coated.

Marinate the steaks in the refrigerator for at least 2 hours, or longer for a stronger flavor.

Place sliced red onions in a microwave-safe bowl, cover with a lid.

Microwave on high for 3 minutes, stirring after 1.5 minutes.

Stir the remaining golden syrup into the microwaved onions.

Season the onions with salt and pepper.

Set the onions aside to cool slightly.

Preheat a BBQ plate until hot.

Grease the hot BBQ plate with the remaining olive oil.

Add the onions to the BBQ plate, tossing frequently for 3-5 minutes until golden brown and caramelized.

Transfer the caramelized onions to a plate and keep warm.

Cook the marinated steaks on the BBQ plate to your desired level of doneness.

Lightly toast the hamburger buns.

Arrange butter lettuce leaves and grated carrots on the bottom buns.

Top with 2 pieces of cooked steak and a generous portion of caramelized onions.

Place the top bun on the burger.

Serve immediately with potato crisps or your favorite sides.