Preheat the oven to 400ºF.

Butter a 7"x11" baking dish or coat with baking spray. Cut enough cauliflower and broccoli into bite-size pieces to nearly fill the dish.

Bring a large pot of water to a boil. Add the vegetables and cook for 2 minutes or until they are cooked but still quite crisp. Drain in a colander and cool under cold running water. Drain again, turning into the baking dish.

Combine the bread crumbs, cheese, basil or parsley, horseradish, salt and pepper in a small bowl. Melt 2 tablespoons of the butter and stir it in. Set aside.

At this point, the vegetables and crumbs can be covered separately and refrigerated for up to a day.

To make the sauce, melt the second two tablespoons of butter in a saucepan over medium heat. Add the chopped onion. Sauté 3 to 5 minutes or until softened. Stir in flour and cook for 1 minute, stirring constantly. Stir frequently for 10 minutes or until the mixture thickens and just boils. Stir in the salt and cheese on low heat. Pour the cheese sauce evenly over the vegetables.

Scatter the crumb mixture evenly over the vegetables. Bake for 20 to 25 minutes or until the vegetable gratin is bubbly around the edges and the topping is crisp and brown.