Cook spaghetti according to package directions.
Meanwhile, in a large stock pot, melt six tablespoons butter over medium high heat. Add onions and sauté until tender. Add mushrooms and red pepper. Sauté 4-5 minutes longer or until vegetables are tender.
Stir in flour and salt until blended. Gradually whisk in milk and bouillon. Bring to a simmer stir for 2 minutes or until thickened.
Stir in turkey, cheese and cayenne pepper; heat through. Season with salt to taste. Remove from the heat.
Drain spaghetti. Add to turkey mixture and mix well. Transfer to one 9-in. x 13-in. x 2-in. greased baking dish or two 8-in. x 8-in. greased baking dishes.
Melt remaining butter. Toss with bread crumbs and sprinkle over casseroles.
Bake, uncovered, at 350° for 30-35 minutes or until heated through and breadcrumbs are golden brown.