Preheat oven to 450 degrees F.
Place swordfish paillard on 4 separate ovenproof plates, cover each with plastic and place in refrigerator. Bring 2 quarts water to boil, add 1 tablespoon salt and set up a handy ice bath in an 8-quart mixing bowl. Drop leek julienne into boiling water and cook until tender, about 1 minute. Remove leeks and plunge into ice bath to cool, about 1 minute. Remove from ice bath, drain well and set aside.
In a small mixing bowl, stir together grapefruit zest and juice, mustard, olive oil and mustard seeds until lightly emulsified. Remove swordfish plates from refrigerator and uncover. Place in preheated oven and cook 30 to 45 seconds, until just opaque. Remove and place on counter. Dress leeks with remaining oil and lemon juice and divide among 4 swordfish plates, making a pile in the center of each. Place 3 grapefruit segments on each plate and sprinkle with zest. Drizzle with sauce and serve warm.