Preheat the oven to 350° F.
Cook spaghetti according to package directions until al dente. Drain the spaghetti and set aside.
Meanwhile, in a large stock pot, melt four tablespoons of butter over medium-high heat. Add the onions and sauté until tender, about 5-7 minutes.
Add the mushrooms and red bell pepper. Sauté 4-5 minutes longer or until vegetables are tender.
Stir in flour and salt until blended.
Gradually whisk in the milk and bouillon. Bring the milk to a simmer and stir for two minutes or until thickened.
Stir in turkey, cheese and cayenne pepper. Heat through. Season with salt and pepper to taste. Remove the pot from the heat.
Add the spaghetti noodles to the turkey mixture and mix well.
Transfer the casserole to one 9-in. x 13-in. x 2-in. greased baking dish or two 8-in. x 8-in. greased baking dishes.
Melt the remaining butter. Toss the butter with the bread crumbs and sprinkle it over the casserole.
Bake, uncovered, for 30-35 minutes or until the casserole is heated through and bread crumbs are golden brown.