Step 1To make the lemon oil, combine the lemon zest and the olive oil. Set aside.
Step 2Thinly slice the tuna (about one-fourth-inch thick), and piece it together into a (5-inch-by-2-inch) rectangle on each of four plates. Drizzle each with about one-half teaspoon lemon oil.
Step 3Shave the fennel across the grain on a mandoline. Divide into four portions (one-fourth cup each) and place on top of the tuna on each plate.
Step 4Using a long, sharp knife, cut the watermelon into 16 very thinly sliced 3-inch-long strips. Lay 4 slices over the fennel on each plate.
Step 5Season with a little sea salt and a drizzle of lemon oil. Garnish the plate with a small pile of black sea salt. Reserve the remaining lemon oil for another use.