In a skillet heat 3 tablespoons of butter; add mushrooms and saute briefly. Melt 4 tablespoons of butter in a saucepan; add flour and stir until smooth. Whisk in broth and cook until thickened and smooth.
Add salt and Tabasco to taste. Whisk a little of the hot sauce into the beaten egg yolk, then pour the egg yolk mixture into the rest of the sauce. Add sherry, cream, turkey, and mushrooms. Cook, stirring, just until heated through.
Cook spaghetti according to package directions. In a buttered casserole, place alternate layers of spaghetti and sauce. Sprinkle with grated Parmesan. Brown quickly under a preheated broiler and serve.