Combine sweet potato, garlic, mince, parsley, zucchini and egg. Mix well with your hands. Shape into four round patties and flatten.

Spray a large frying pan with oil and place over medium heat. Cook patties for 5 minutes each side, or until cooked through.

Halve rolls and toast. Top each with watercress, onion, burger, tomato, avocado and sweet chilli.