How to Make It
Using a mandoline, very thinly slice the green tomatoes and white onion. Layer the slices in a heatproof 4-cup measuring cup. In a small saucepan, combine the water with the vinegar, garlic, peppercorns, salt and sugar and bring to a boil. Pour the hot brine over the tomatoes and onion and let cool to room temperature, about 1 hour. Cover and refrigerate for 1 hour, then drain and serve.