Preheat oven to 375°F. Grease 9x13 baking dish.
Cook pasta according to package directions, adding chopped cauliflower in last 5 minutes. Drain.
Meanwhile, melt butter in pot over medium heat. Add flour, whisk 1 minute.
Gradually whisk in milk. Cook, stirring constantly, until thickened.
Remove from heat. Stir in seasonings and 2 cups of cheese mixture until melted.
If using, stir in grated carrots and zucchini.
Combine sauce with pasta and cauliflower. Transfer to baking dish.
Mix breadcrumb topping, sprinkle over pasta.
Bake 15 minutes until bubbly and golden.