Place the tuna, onions, mustard, eggs, lemon juice, salt, pepper and 1/4 cup of the bread crumbs into a medium mixing bowl and stir to combine. Divide the mixture into 8 rounds and set aside on a parchment lined half sheet pan. Allow to rest for 15 minutes. Place the remaining bread crumbs into a pie plate. One at a time, coat each round in the panko on all sides.