Drain the salmon, reserving 3/4 cup of the liquid. Flake the meat. Melt butter in a large skillet over medium high heat. Add onion, and cook until tender.
In a medium bowl, combine the onions with the reserved salmon liquid, 1/3 of the cracker crumbs, eggs, parsley, mustard and salmon. Mix until well blended, then shape into six patties. Coat patties in remaining cracker crumbs.
Melt shortening in large skillet over medium heat. Cook patties until browned, then carefully turn and brown on the other side.