Cut fish into 8 equal pieces, discarding any bones you find. Spread the panko on a wide plate; mix in 1/2 tsp. of the seasoning salt. Set a piece of the fish on the panko. Firmly press a thin layer of the panko on both sides of the fish. Set the fish on a plate. Coat the remaining pieces of fish in this fashion.
Preheat oven to 425 degrees F. Cut each potato in half lengthwise. Cut each half into 6 wedges. Place wedges in a bowl and toss with oil and remaining seasoning salt. Arrange potatoes in a single layer on a large parchment paper-lined baking sheet.
Bake 15 minutes, and then turn each potato over and bake 5 minutes more. Make a spot for each piece of fish amongst the potatoes and set them in place. Bake 8 minutes more, and then turn each piece over. Baked 5 to 7 minutes more, or until fish are cooked through and the potatoes crispy. Serve the fish and potatoes with lemon slices and, if desired, tartar sauce.
Note: A range of ready-to-use seasoning salt can be found in the aisle where bottled spices and herbs are sold.