Prepare the shrimp: Combine 8 cups cold water, 2 tablespoons salt, the coriander and peppercorns in a large saucepan. Squeeze in the lemon juice and add the wedges. Boil 5 minutes, then remove from the heat, add the shrimp, cover and set aside, about 15 minutes. Strain and transfer the shrimp to a bowl; cover and chill at least 1 hour.
Meanwhile, make the sauce: Preheat the oven to 350 degrees F. Place the tomatillos, onion, garlic and jalapenos in a roasting pan. Toss with the canola oil and season with salt and pepper. Roast until soft, tossing occasionally, 25 to 35 minutes.
Cook the spinach in a saucepan of boiling water, about 1 minute. Drain, squeeze dry and transfer to a food processor. Add the tomatillo mixture, vinegar, horseradish, cilantro and honey and pulse until smooth; season with salt and pepper. Transfer to a bowl, cover and chill at least 1 hour. Bring to room temperature 15 minutes before serving. Serve the sauce and shrimp in shot glasses, if desired.