Rinse the beef and pat dry with paper towels.
Slice the sirloin in thin pieces.
Slightly grease a non-stick skillet with cooking spray.
Heat it over medium-high heat and cook the beef for 2 minutes on each side.
Sprinkle some salt and pepper while cooking.
When it's done, remove the meat from the skillet.
Cut the bell peppers in thin strips and chop the mushrooms. Grate the ginger.
Heat 2 tbsp olive oil over medium-high heat.
Saute the peppers for 2 minutes.
Add the ginger and cook for 2 minutes more, stirring constantly.
Stir in the mushrooms and cook for 3-4 minutes more until they begin to soften.
Season with salt, pepper and add the Cayenne pepper. Stir well.
Bring the meat back to the skillet and add the balsamic vinegar. Cook for 1 more minute and remove from heat.