In a skillet cook the shallots in the olive oil over moderate heat, stirring, until they are softened, stir in the mustard seeds and the curry powder, and cook the mixture, stirring, for 1 minute. Stir in the sugar, the tomatoes, and salt and pepper to taste and cook the mixture, stirring, for 2 minutes, or until the tomatoes begin to release their juices. Stir in the balsamic vinegar and keep the coulis warm. Heat a well-seasoned ridged grill pan over moderately high heat until it is hot and brush it with vegetable oil. Add the scallops, patted dry, and grill them for 2 1/2 minutes on each side, or until they are just firm. Divide the coulis between 2 plates and top each serving with half the scallops.
