Mix all the burger ingredients together and shape into 8 burgers approximately 1 1/2 cm thick. Fry a teaspoon of the mixture in a tablespoon of vegetable oil to check the seasoning. Add more salt and pepper to taste as necessary. Cover and chill for 10 minutes.
Fry, grill or BBQ the burgers for 5 minutes per side or until cooked through.
To assemble your burgers
Generously spread the two halves of each roll with mustard mayonnaise, place a lettuce leaf onto the base of each roll then add the burger and arrange 1 or two onion slices on top. Gently press the top half of roll in place and serve immediately.