Sauté mushrooms and green pepper in 2 tablespoons butter in a small saucepan until tender. Set aside.
Melt remaining butter in a heavy saucepan over low heat; add flour, and stir until smooth. Cook 1 minute, stirring constantly. Combine milk and chicken broth; gradually add to flour mixture, stirring constantly. Cook over medium heat, stirring constantly, until thickened and bubbly. Stir in sautéed vegetables, chicken, and pimiento.
Beat yolks. Gradually stir one-fourth hot mixture into yolks; add to remaining hot mixture, stirring constantly.
Combine remaining ingredients, except patty shells; stir into creamed mixture. Bring to a boil; reduce heat, and simmer 10 minutes. Spoon mixture into patty shells. Serve immediately.