1) In a 9-inch glass pie plate, melt the butter in the microwave for 20 seconds.
2) Stir in the scallops and onions, then cover with a plastic wrap.
3) Cook for 2 1/2 to 3 1/2 minutes or until the scallops are almost opaque. Stir once and reserve 2 tablespoons liquid.
4) In a bowl, mix the cornstarch, curry powder, salt and 2 tablespoons reserved liquid together.
5) Stir into the scallops and cover with plastic wrap.
6) Cook for 2 minutes, or until the sauce is thickened and creamy.
7) Spoon the scallops over rice, garnish with the tomatoes and watercress and serve immediately.