1In 5-quart Dutch oven, cook and drain spaghetti as directed on package. Return to Dutch oven; cover to keep warm.
2Meanwhile, in 10-inch nonstick skillet, melt 2 tablespoons of the butter over medium heat. Cook mushrooms in butter 4 to 6 minutes, stirring frequently, until light brown. Add broccoli, bell pepper, garlic and water. Cover; cook 3 to 4 minutes, until vegetables are crisp-tender. Remove vegetables from skillet to Dutch oven.
3In same skillet, melt remaining 2 tablespoons butter over medium heat. Add flour; cook 2 minutes, stirring to make a smooth paste. Add 1 cup of the half-and-half; stir until smooth. Gradually add remaining 1 1/2 cups half-and-half; heat just to boiling, stirring constantly. Remove from heat; stir in salt and sherry.
4Pour sauce over spaghetti and vegetables. Stir in cheese. Cover; let stand 5 minutes.