In a medium bowl, mix tuna, organic Greek yogurt, celery, red onion, pickles, dill, and mustard. Season with salt, pepper, and 1 teaspoon of pickling liquid.
Spread both slices of sourdough with a spoonful of yogurt. On one half, add 1 slice cheese (to the yogurt side of bread), tuna mixture, tomato slices, and another slice of cheese. Top with remaining slice of bread, yogurt side on the inside of the sandwich.
Heat a thin layer of oil in a cast iron or nonstick skillet over medium heat. Add sandwich and cook until bread is golden brown on both sides, about 4 minutes per side. Transfer to a cutting board. Slice in half and serve immediately.
Thanks to Straus Family Creamery for sponsoring this post!