Place London broil along with mushrooms, wine, parsley, rosemary, garlic and shallots in a plastic bag and secure
Place in the refrigerator for two to three hours to marinate
Remove beef from marinade and place in a large skillet and cook over medium heat until cooked to your liking
Place steak aside
In the same skillet, place reserved marinade, beef broth and tomatoes and cook over low heat for 20 minutes or longer until reduced and slightly thickened
If you want a thicker sauce, remove a half cup of the marinade mixture, add cornstarch and blend until smooth
Then pour the cornstarch mixture back into the remaining sauce and continue to cook until thickened
Slice London broil and pour sauce over meat