Cauliflower and Broccoli Cheddar Gratin

1 Butter a 9 x 13 baking dish.
2 Cut enough cauliflower and broccoli into bite-size pieces to nearly fill the dish.
3 Peel and thinly slice the carrot.
4 Bring a large pot of water to a boil.
5 Add the vegetables; cook 2 minutes or until cooked but still very crisp.
6 Drain in a colander; cool under cold running water.
7 Drain again; turn into baking dish.
8 Combine the bread crumbs, cheese, basil, horseradish, salt and pepper in a small bowl.
9 Stir in the butter; set aside.
10 To make the sauce, melt butter in a medium saucepan over medium heat.
11 Add the chopped onion; saute 3 to 5 minutes or until softened.
12 Stir in the flour and cook for 1 minute, stirring constantly.
13 Gradually mix in the milk and mustard while stirring constantly.
14 Cook for 10 minutes, stirring frequently, until the mixture thickens and just boils.
15 Remove from heat; stir in the salt and cheese; pour evenly over the vegetables.
16 Arrange the rack over the oven centre; preheat the oven to 400 degrees F.
17 Scatter the crumb mixture evenly over the vegetables in cheese sauce.
18 Bake 20 to 25 minutes or until bubbly around the edges and the topping is crisp and brown.