Prepare Onions
Pearl onions can be difficult to peel so it is best to boil them in salted water for about 5 minutes. (reserved water)  Allow them to cool enough to handle then peel with a small pairing knife.

Cook Sprouts
Using the same cooking water add the Brussels sprouts and boil for about 6 minutes or until just tender.  Drain, allow to cool and cut each sprout in half lengthwise. (Store tightly covered, refrigerated if making ahead)

Prepare Sauce and Final Dish
Whisk together 3 tablespoons horseradish, flour, and allspice in small bowl to blend; whisk in whipping cream. Melt butter in heavy large skillet over medium heat. Add chopped thyme and stir briefly.

Add  the onions and Brussels sprouts and sauté until heated through, about 4 minutes. Add cream mixture and cook until vegetables are coated and cream mixture is reduced to glaze, stirring often, about 3 minutes.

Season to taste with salt and pepper and add more horseradish to taste if you like.

Transfer to a serving bowl and garnish with thyme sprigs.