In a large bowl, combine the cooked pasta, onion, celery, peas, and tuna. Squeeze the lemon juice over the contents of the bowl (being careful not to drop seeds in). Sprinkle with the salt and pepper, to taste.
In a smaller bowl, mix together the Greek yogurt, Cheddar cheese, spicy mustard, red wine vinegar, parsley, and lemon pepper. Pour the sauce over the pasta/tuna mixture in the larger bowl and stir or toss to coat.
If you are serving this casserole cold, empty the contents into your serving dish, sprinkle the top with the Panko breadcrumbs, and refrigerate for at least an hour (to let the flavors meld).
If you are serving this hot, then preheat the oven to 350 degrees F and lightly grease a 9 x 13 casserole with non-stick cooking spray or vegetable oil. Spread the noodle mixture evenly in the prepared pan and sprinkle the Panko bread crumbs over the top. You can also sprinkle with a little extra cheese if you like.
Bake for 20 to 30 minutes, just until it’s heated through and the sauce begins to bubble a little.