Arrange carrot, broccoli, and cauliflower in a steamer basket over boiling water. Cover and steam 6 to 8 minutes or until vegetables are crisp-tender. Drain well.
Coat a large nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add mushrooms and garlic; saute until tender. Add vegetables, tarragon, and Worcestershire sauce; stir well.
Combine milk and sour cream, stirring until smooth. Add to vegetable mixture, and cook until thoroughly heated. (Do not boil.) To serve, place 1 cup pasta on each serving plate; spoon 1 cup vegetable mixture over each serving. Sprinkle with Parmesan cheese and freshly ground pepper. Garnish with fresh basil sprigs, if desired.