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.