In a large saucepan on the stovetop, cover and steam the cauliflower and carrots, with enough water to keep the pan from going dry, for 3 minutes.
3 cups cauliflower, , 3 cups carrots,
Add the broccoli. (Make sure that there is still enough water.) Cook for another 4- 6 minutes until the vegetables are tender-crisp. Drain the vegetables.
3 cups broccoli,
Meanwhile, preheat the oven to 350℉ (180℃).
Combine the mayonnaise, chopped onion, prepared horseradish, salt, and pepper.
1 cup mayonnaise, ¼ cup onion,, 3 tablespoons prepared (creamed) horseradish, ¼ teaspoon fine sea salt, ⅛ teaspoon ground black pepper
Mix the drained vegetables with the mayonnaise blend to evenly coat them. Transfer to a greased, 3-quart casserole dish.
Combine the breadcrumbs, melted butter, and paprika. Sprinkle this mixture over the vegetables in the casserole dish.
⅓ cup dry breadcrumbs,, 2 tablespoons butter,, ⅛ teaspoon paprika
Bake uncovered for 25 - 30 minutes or until heated through.
Notes
If you have assembled the casserole ahead of time and kept it in the fridge, it will need to bake a little longer to be heated through.