How to Make It
In a medium saucepan, bring the vinegar, brown sugar and salt to a boil, stirring. Remove from the heat.
In a medium skillet, heat the oil. Add the garlic, grated ginger, mustard seeds, black pepper, turmeric, ground cumin and cayenne pepper and cook over low heat until fragrant, about 2 minutes. Carefully pour the hot oil into the vinegar mixture.
In a large heatproof bowl, combine the tomatoes, scallions and jalapeños. Stir in the hot pickling liquid and let stand at room temperature for 4 hours or refrigerate for 8 hours, then serve.
Grilled steak or fish.