Preheat the oven to 450° F.

Place the swordfish paillards on 4 individual ovenproof plates, cover with plastic, and refrigerate. Bring about 2 quarts of water to a boil, add about 1 tbsp salt, and set up a handy ice bath.

Drop the julienned leeks into the boiling water and cook until tender, about 1 minute. Remove the leeks and plunge into the ice bath to cool, about 1 minute. Drain well and set aside.

Remove the zest from the grapefruit with a zester or a sharp knife, then cut off any remaining skin and pith. Holding the grapefruit over a bowl to catch any juice, cut between the membranes to free the individual segments.

In a small mixing bowl, whisk together half the grapefruit zest, the grapefruit juice, mustard, 1/4 cup of the olive oil, and the mustard seeds.

Remove the swordfish from the refrigerator and uncover. Place in the oven, plates and all, and cook 30 to 45 seconds, until fish is just opaque.

Dress the leeks with the remaining 2 tablespoons oil and the lemon juice and make a pile in the center of paillard. Arrange 3 grapefruit segments on each plate and sprinkle with the remaining zest. Drizzle with the sauce and serve.