If using flank steak, follow same directions.
If using diced chicken, pat dry and season with a pinch of salt and pepper. Follow same instruction as steak strips in step 2, stirring occasionally until chicken reaches minimum internal temperature, 5-7 minutes.
If using ground beef, cook before mushrooms. Place a large non-stick pan over medium-high heat. Add ground beef, 1/4 tsp. salt, and a pinch of pepper to hot pan. Stir occasionally, breaking up meat, until no pink remains, 4-6 minutes. Remove ground beef to a plate, and wipe pan clean. Cook mushrooms, returning ground beef to the pan with stroganoff sauce.