1. Oil grill and heat barbecue to medium. In a bowl, whisk the egg with sour cream and Dijon. Stir in bread crumbs, seasonings and onion. Crumble in meat. Using a fork or your hands, gently mix. Divide into 4 portions and shape into burgers, each about 1/2 in. (1 cm) thick. If making ahead, place on a plate.
2. Place burgers on barbecue. Grill with lid closed until meat is firm to the touch and completely cooked through, 6 to 8 min per side.
3. Top with cheddar slice and sweet chilli sauce, then serve on lightly grilled Wonder® hamburger buns.