Drain the liquid from the tuna cans and reserve a tablespoon of the tuna water.
In a medium bowl, mix together the tuna, mustard, torn white bread, lemon zest, lemon juice, water, parsley, and onions. Add salt and freshly ground black pepper to taste. Mix in the egg.
Divide the mixture into 4 parts. With each part, form into a ball and then flatten into a patty. Place onto a wax paper lined tray and chill for an hour.
Heat the olive oil and a little butter in a cast iron or stick-free skillet on medium high. Gently place the patties in the pan. Cook until nicely browned, 3-4 minutes on each side.
