Using a fine bladed knife (a fish filleting knife if you have one) cut the tuna into very fine slices (about the thickness of smoked salmon).
Put the lemon juice into a small dish. Remove the coarse outer leaves from the fennel and trim the tops. Cut the fennel in half and remove the hard core but not so much of it that the pieces fall apart.
Now, using a very sharp small knife or a mandolin, cut the fennel into wafer thin slices (they should be almost transparent if you can manage that). As soon as you cut them put them into the bowl with the lemon juice (otherwise the fennel will discolour).
Lay the slices of tuna and fennel onto two plates (you can put the fennel first, or the tuna first, whichever you prefer), sprinkle with the salt and pepper and drizzle the olive oil over each serving.