1) Place the tuna, onions, mustard, eggs, lemon juice, salt, pepper and 1/3 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.
2) Heat enough olive oil to cover the bottom of a 30cm saute pan over medium heat until shimmering. Add the croquettes and cook 2 to 3 minutes on each side or until golden brown. Remove to a cooling rack set over a half sheet pan lined with paper towels. Allow to cool for 2 to 3 minutes before serving.