Cut the prawns halfway through, slitting from the head down the back to the tail. De-vein the prawns, lay them on a baking sheet, and split open, flesh side up. Sprinkle the spice mix evenly over the prawns.

Cut the skin and pith from the fruits. Over a bowl, segment the fruits, catching both the segments and the juices. Squeeze the cores to extract any extra juice. Season with salt and pepper to taste and add 2 tablespoons of the olive oil. Toss well, taste for balance, and reserve.

Preheat the oven to 400°F.

In a large skillet with curved sides, heat the remaining 2 tablespoons olive oil over medium heat. Lay the prawns, flesh down, against the curved sides of the pan, tails up and heads toward the center. Press them down with a spatula or rest a slightly smaller pan on top of the prawns to weight them, so they cook evenly and rapidly. This will give them a gently curled shape. Cook about 2 minutes longer, until they turn pink. Place the pan in the oven until the prawns are evenly pink, about 2 minutes longer.

To serve, arrange the prawns on a platter, standing them tails up. Spoon the fruits in the middle of the arrangement, and place the watercress on top. Serve hot or at room temperature.