Slice the scallion greens into long, thin ribbons (i.e. along the length of the stalk). Fill a bowl with ice water, and place the scallions in the water. They should curl up and be ready to use by the time you're ready to serve.

Dice the watermelon into small cubes (I went with about 1 cm square, but you can go with whatever size you think will work best). Set aside.

Using a very sharp knife, cut the tuna into small cubes or small pieces (cubes are nice for presentation, but any small bite-sized pieces will work). Set aside, separate from the watermelon. (Note - if you're having trouble cutting the tuna, put it in the freezer for 20 minutes and then cut it, but be careful not to leave it too long or you'll affect the texture).

Place the fennel seeds, peppercorns, and salt in a spice grinder or mortar and pestle and pulverize. Toss the spice blend with the tuna and set aside to cure for 7-10 minutes.

While the tuna is curing, combine the olive oil, lemon juice, vinegar, and shallots in a small bowl. Toss the mixture with the watermelon and let stand for 5 minutes.

After the curing time has passed, combine the tuna and watermelon gently in a serving bowl. Take care not to crush the watermelon or shred the tuna. Taste and adjust the salt, if necessary. Chill in the refrigerator for 3-4 minutes, then serve with slices of crusty bread and topped with scallions.