Melt a knob of butter in a pan over a medium-high heat and cook ½ a chopped onion for 3-4 minutes until just softened. Allow to cool to room temperature. In a large bowl, combine the cooled onion with 450g beef mince, 1 large egg, 4 tbsp breadcrumbs, 1 tbsp Worcestershire sauce, ½ tsp dried thyme and some salt and black pepper. Mix until just combined. Divide the mixture into four equal portions and gently shape each into a patty 8-10cm wide. Use your thumb to make a dimple in the centre of each burger. Cover and refrigerate for one hour. Cook the burgers on a medium-hot barbecue. Leave them undisturbed for the first 4-5 minutes so they build up a good seal, then carefully turn them over. Cook for a further 4-5 minutes or until no longer pink inside.