Bake the fish according to the package directions. Wrap the stack of tortillas in foil and place in the oven during the last 5 minutes of baking. To assemble the tacos, place the warm tortillas on serving plates, put 2 tenders on each tortilla, and cover with a few slices of the onion. Add a dollop of the tartar sauce and top with 1/4 cup of the lettuce. Fold over and serve.