Boil carrots in water until just tender reserving cooking liquid. Mix mayonnaise, chopped onion, 2 tablespoons horseradish or up to the entire bottle to taste (we use the entire bottle), fresh ground black pepper (to taste) and a bit (about a half cup) of the carrot cooking liquid to make a sauce. Fold cooked carrots into sauce and pour into baking dish sprayed with non-stick spray. Combine crushed crackers with melted butter to make topping. Sprinkle topping onto casserole and bake uncovered in 350 degree oven for about 30-45 minutes or until bubbley and golden brown.