Trim tendrils off prawns with scissors. Using a small, sharp knife, cut a shallow slit along the back of each prawn and remove the vein. Rinse under cold running water. Drain and pat dry with paper towels.

In a bowl, mix together cornstarch and salt. Thoroughly coat each prawn with the cornstarch mixture.

Heat oil in a wok or a large sauté pan over high heat. . Add prawns and sauté until just turning pink, about 2 minutes. Add garlic, ginger, green onions and crushed peppercorns. Cook, stirring constantly, until prawns are cooked through, about 3 minutes more.

Serve, garnished with green onions.