Crumble egg yolk; finely grate egg white, and set aside.
Melt 1 1/2 tablespoons butter in a saucepan over low heat. Add mushrooms and green pepper; sauté until tender. Set aside.
Melt remaining butter in a large skillet over low heat; add flour, and stir until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly.
Add crumbled yolk, chicken, salt, pepper, pimiento, and sautéed vegetables to white sauce. Spoon mixture into toast cups; sprinkle with grated egg white. Garnish with parsley, if desired.