Combine the breadcrumbs and milk in a large bowl and stand for 10 minutes. Add the beef, rosemary, garlic and salt then mix until just combined. Divide and shape into 12 small burgers. Chill for 1 hour

Combine the mayonnaise, tomatoes and extra garlic in a food processer and process until smooth. Chill until required

Cook the burgers on a barbeque for 3 minutes each side or until cooked through. Place some spinach and avocado onto the base of the rolls, top with the burgers and onion. Spread the top of each roll with the mayo then sandwich together. Serve immediately