Preheat the oven to 350°. Place the tomatillos, onion, garlic and jalapeño in a small roasting pan. Toss with the oil and season with salt and pepper. Cook until the tomatillos are soft, 20 to 25 minutes. Transfer to a food processor and puree until smooth. Add the vinegar, horseradish and cilantro and pulse just to combine. Season with salt and pepper. Scrape the sauce into a bowl, cover and refrigerate for at least 1 hour before serving.