In a large mixing bowl combine yogurt, mustard, lemon juice, sea salt and pepper. Whisk well to combine.
Chop your celery, onion, parsley and tarragon really small, then add into the bowl with the yogurt dressing.
Add tuna and toss everything really well to coat.
Cut the stems off the bell peppers. Using a paring knife, remove the remaining pith and seeds.
Turn the pepper upside down to shake any loose seeds out.
Next cut the pepper in half vertically through the middle of the stem to create little "boats", as shown.
Spoon your tuna salad into the bell pepper halves and serve immediately.