Put all the ingredients, except the breadcrumbs, in a bowl and knead together. Gradually add the breadcrumbs until the desired amount of moisture has been absorbed then add the feta.
Shape into patties and arrange on a greaseproof paper-lined baking tray. Bake at 200 degrees C, grill setting, for approximately 20΄
Slice the buns in half and fill with the burgers, mustard, ketchup and mayonnaise.