Combine the tomatoes, scallions, mint, ginger, and lemon or lime juice in a medium bowl.
Heat the vegetable oil in a small pan or skillet and add the mustard seeds. When the seeds begin to pop (have a lid handy to contain them), remove the pan from the heat. Pour the oil and seeds over the tomato mixture, add salt to taste, and lightly toss. Let the salad marinate for about 30 minutes before serving.