Mix coriander, 3 tbsp. oil, 2 tsp. paprika, the salt, and pepper in a bowl. Rub roast all over with mixture and chill, uncovered, at least 24 hours and up to 4 days.
Preheat oven to 250°. Heat 1 tbsp. oil in a large dutch oven over medium-high heat. Brown roast on all sides. Rub onions with a little oil and the paprika; set aside.
Drain wood chips and transfer to a roasting pan just big enough to fit roast. Spread chips evenly in pan and set a metal rack over chips. Set roast on rack, spoon onions around it, and cover with heavy-duty foil, sealing tightly.
Roast meat until it registers 110° on an instant-read thermometer, 1 1/2 to 2 hours. Increase oven temperature to 475°, uncover, and roast until meat registers 130°, about 20 minutes. (If wood chips look dry, sprinkle with a few tbsp. water.) Transfer roast and onions to a carving board and let rest, uncovered, 20 minutes.
Mix sour cream and horseradish together in a serving bowl. Slice roast thinly and serve with horseradish cream and onions.
Note: Nutritional analysis is per serving.