Sauté the onion and 1 cup of the chestnuts in 1/4 cup butter. Stir in the flour and cook, stirring, about 2 minutes. Remove from heat and gradually add the broth and milk. Return to heat and cook, stirring, until thickened. Season to taste with salt and white pepper.
Cook the broccoli, cauliflower, and carrots in boiling water until crisp-tender. Drain. Combine with the cream sauce and spoon into a casserole dish.
Sauté the remaining 1/4 cup chestnuts, rosemary, and bread crumbs in remaining 2 Tbsp butter. Sprinkle over the top of the casserole. Bake at 350° F or 30 minutes, or until heated through.
Can be prepared ahead, but sprinkle the casserole with bread crumbs just before baking. Increase the baking time if the vegetables have been refrigerated.
Servings: 8