Shave the fennel and onion on a mandoline or slice very finely. Cut the chile up to the stalk without separating the halves. Halve the garlic across the middle, leaving the skin on; you will use one half. Put half the onion and fennel with the fennel seeds, coriander seeds, bay leaves, chile and peppercorns in a small pan. Lay both halves of the divided tuna loin on top. All should fit snugly.