Bring 1 cup water to a boil in a saucepan. Add carrot and cauliflower; cover and simmer 8 minutes. Add brussels sprouts, and cook 8 minutes or until tender; drain and keep warm.
Melt butter in a skillet over medium heat; add onion, and sauté until tender. Whisk in flour until smooth; cook, whisking constantly, 1 minute. Gradually add milk; cook over medium heat, whisking constantly until thickened. Stir in mustard and next 3 ingredients; toss with vegetables.