How to Make
Place mayonnaise, sour cream, lime juice and chili powder in a small bowl, whisk until the ingredients are fully combined. Store in an airtight container until ready to use.
Heat oil in a large heavy based pan. Add onion slices and a pinch of salt, fry for 5 minutes until translucent, lower heat and fry for 15 minutes until caramelized.
Spray barbecue grill with oil. Heat on medium high heat. Cook patties for 5 minutes each side until cooked through and top with cheddar cheese. Toast burger bun on grill.
To assemble the burgers, spread a few tablespoons of avocado on the bottom bun and place burger on top. Cover with carmelized onion, chipotle mayonnaise and rocket leaves. Serve immediately.