Sprinkle the fennel spice evenly over the flesh side of the prawns and let sit, covered and refrigerated, overnight.
Preheat the oven to 400 degrees F.
Heat 2 tablespoons of the olive oil in a large skillet with even curved edge over medium high heat until hot. Lay the prawns, flesh down, against the curved sides of the pan, tails up and heads toward the center of the pan. Press them down with spatula or rest a slightly smaller pan on top of the prawns as a weight, so they cook evenly and rapidly. This will give them a gently curled shape. Cook until they begin to turn pink around the edges, about 2 minutes. Place the pan in the oven until the prawns are evenly pink, about 2 minutes longer.
Cut the skin and white pith off all the citrus fruits and segment the fruits over a bowl to catch the segments and juices. Season with the salt and pepper and add the remaining 2 tablespoons of the olive oil. Toss well, taste for balance and reserve.
To serve, place 1/4 of the watercress on 4 plates. Toss the citrus mixture again and spoon around and over the watercresss, moistening with juices. Arrange 3 prawns around each salad, standing them in a circle, tails up, and serve hot, warm or at room temperature.
Fennel Spice Rub:
This is my favorite spice mixture. There is almost nothing it doesn't taste good on or in. Use it to make spice-encrusted pork ribs, chops, or tenderloin; veal chops; chicken breasts; duck; beef; liver; or eggplant; or add a teaspoon to lentil soup. If you don't have time to make it, we do it for you, with our Fennel Spice Rub
1 cup fennel seeds
3 tablespoons coriander seeds
2 tablespoons white peppercorns
3 tablespoons kosher salt
Put the fennel seeds, coriander seeds, and peppercorns in a heavy pan over medium heat. Watch carefully, tossing frequently so the seeds toast evenly. When light brown and fragrant, pour the seeds onto a plate to cool. They must be cool before grinding, or they will gum up the blades.
Pour the seeds into a blender and add the salt. Blend to a fine powder, shaking the blender occasionally to redistribute the seeds. Store in a tightly sealed glass jar in a cool, dry place, or freeze.
Yield: about 1 1/4 cups
Prep Time: 10 minutes