How to Make It
Preheat the oven to 350°. Spread the tomatillos, garlic, jalapeño and half of the chopped onion in a small roasting pan. Toss with the oil and season with salt and pepper. Roast for about 25 minutes, until the tomatillos are softened. Transfer the mixture to a food processor and puree until smooth. Add the rice vinegar, horseradish and cilantro and pulse until blended. Transfer the sauce to a bowl, cover and refrigerate until well chilled, about 1 hour.
Meanwhile, in a medium saucepan, combine 2 quarts of water with the coriander seeds, peppercorns, the remaining chopped onion and 2 tablespoons of salt. Squeeze the lemons into the water and add the wedges to the pot. Bring to a boil over moderately high heat and simmer for 5 minutes. Remove the saucepan from the heat and add the shrimp. Cover and let stand for 15 minutes, until the shrimp are white throughout. Drain well and discard the spices, onion and lemons. Transfer the shrimp to a bowl, cover and refrigerate until well chilled.
Arrange the shrimp on a platter and serve with the tomatillo-horseradish sauce.