Cut the French roll in half lengthwise and toast under the salamander or broiler, cut-side up, until golden-brown (about 15 seconds).
To plate, spread ½ oz. of the tonnato dressing on both the top and bottom halves of the sandwich, spreading to cover completely. Add the arugula to the top half of the sandwich, letting it stick to the tonnato spread. On the bottom portion, lay down the bruschetta mix and shingle on pieces of the seared rare tuna. Add pieces of the tuna conserva (canned tuna) interwoven throughout.
Garnish with the minced pickled red onion, dill and chives. Add celery leaves if you have them available. Serve immediately.