Burgers with pickled green tomatoes

Ingredients

• 1/2 cup cider vinegar
• 2 tbsp granulated sugar
• 1/8 tsp coriander
• 1/8 tsp dry mustard powder
• 1/8 tsp salt
• 2 green tomatoes , seeded, cut into 1/8 inch rounds
• 2-Jan red onion , cut into 1/8 inch rounds
• 1 egg
• 1 tsp Dijon mustard
• 1 tsp Worcestershire sauce
• 1/2 tsp salt
• 500 g medium ground beef