Place the oil, thyme, rind of the lemon peeled into strips, bay leaves and crushed garlic in a pan and bring to a gentle boil over medium-high heat. Then remove the pan from the heat and add the trimmed fish. Let stand 20 minutes. Then remove the fish, reserving the poaching oil, cool and flake.
In a bowl, combine the juice of the lemon with 1/3 cup reserved poaching oil. Whisk to combine and season with salt and pepper. To the dressing, add the celery, onions, hot and sweet peppers, olives, parsley, capers and tarragon. Season the fish with a little seafood seasoning and add to the salad. Combine with the tines of a large fork, cover and store.
To serve, place the rack in the center of the oven and preheat the broiler.
Shred the Cheddar or Gruyere cheese on the large side of a box grater.
Pile the tuna salad on the bottom of the rolls. Top with the cheese. Place both the bottoms and tops under the broiler to toast and melt the cheese, 2 minutes. Serve hot.