Cut the flank steak lengthwise into 2 inch wide strips. Cut crosswise into 1/2 inch lengths. Season with salt and pepper.
Heat oil over high heat in a large skillet. Cook 1/2 the meat, stirring and shaking to cook evenly, for 1 to 2 minutes; remove. Cook remaining meat, shaking and stirring, 1 to 2 minutes. Set aside with first batch.
Melt butter in skillet, add green peppers and mushrooms. Cook until mushrooms start to wilt. Add shallots and cook another minute. Add 1/4 cup sherry. When it starts to boil add beef broth. Heat 1 minute and add cream.
Blend corn starch with remaining sherry and stir into sauce. Stir and return meat to skillet. Season with salt and pepper if needed. Serve.