Slice tomatoes into 6 wedges each. Reserve in a large bowl with scallions and serrano chiles.
In a saucepan bring vinegar to a boil. Add sugar and salt, and cook until dissolved, about 1 minute. Remove from heat and reserve.
Measure ginger, garlic, mustard seeds, cracked peppercorns, cumin, cayenne, and turmeric onto a plate and place near stove. In another saucepan heat oil until smoking. Add spices and cook, stirring constantly with a wooden spoon, until aromas are released, about 2 minutes. Remove from heat and stir in vinegar mixture.
Immediately pour over reserved vegetables. Mix well, cover with plastic wrap, and refrigerate a minimum of 3 days.
c.1997, M.S. Milliken & S. Feniger, all rights reserved