Combine green pepper, mushroom, onion, salt, and water in a small saucepan. Bring to a boil. Reduce heat, and cook over low heat 20 minutes. Strain; set vegetables and reserved liquid aside.
Melt butter in top of a double boiler over medium heat; add flour, and cook 1 minute, stirring constantly. Gradually add reserved vegetable liquid, beef extract, beef broth, and tomato juice, blending until smooth. Add reserved vegetables, olives, and Worcestershire sauce. Cook over medium heat 10 minutes or until sauce is thickened and bubbly, stirring frequently. Serve sauce hot with roast beef.