How to Make It
In a nonreactive saucepan, heat the vinegar, sugar and salt, stirring constantly just until dissolved. Remove from the heat. In a medium bowl, combine the tomatoes, scallions and serranos.
On a plate, arrange the ginger, garlic, mustard seeds, cumin, peppercorns, crushed red pepper and turmeric. In a medium saucepan, heat the oil until almost smoking. Add the garlic, ginger and spices all at once and stir over moderate heat until fragrant, about 1 minute. Remove from the heat and add the vinegar mixture. Add the dressing to the tomatoes and scallions and let cool. Cover with plastic wrap and refrigerate for at least 2 hours or up to 4 days.