Place the tuna steaks in a zip-top bag. Add 2 tablespoons canola oil, pickle juice, ½ teaspoon salt, pepper and celery salt into the bag. Toss to coat and allow flavors to blend for 20 minutes at room temperature.
Heat the grill to medium-high heat. Place the steaks on the grill. Grill for 6 minutes. Don’t move the steak while it’s cooking. It will sear and easily flip when ready.
After about 6 minutes, flip and grill for another 6 minutes. Remove from grill and let cool at room temperature.
In a bowl, whisk together the mayonnaise, mustard, lemon juice, vinegar and ½ teaspoon salt.
Break the tuna steaks up with your fingers and add to the mayonnaise mixture. Stir to coat.
Chop the hard-boiled egg and add to the tuna with the celery, pickle and onion.
Enjoy on a bed of lettuce or as a sandwich.