I love to make Stir Fry because it is so easy. By marinating a round steak or sirloin steak for sometime the meat becomes very tender.
Cut the meat into thin strips, across the grain, and place in the marinade for at least an hour. Overnight is even better. 
Turn the meat occasionally. 
In a wok or deep frying pan heat oil until hot and then add the meat and brown. 
Add the onions, and mushrooms if desired, and cook until tender. 
Add the broccoli and continue to turn the meat and vegetables to soften. 
Finally add the zucchini, add the marinade with cornstarch added and cover to steam. 
When sauce is thickened remove from the heat. 
Serve over hot rice.
Stir Fry Beef and Vegetables
1 lb. lean beef (sirloin, top round) sliced thinly across the grain ½ c. soy sauce
¼ c. water
1 c. onions, thinly sliced
1 c. celery, sliced diagonally
2 c. broccoli florets
2 c. broccoli stems peeled and slice thin
½ c. vegetable oil
2 cloves garlic, minced
12 medium mushrooms, sliced
1 T. cornstarch prepared rice
Marinate beef in water and soy sauce for at least 30 minutes. Prepare vegetables and set aside. In a wok or large frying pan cook drained meat (reserve the marinade for later) over low heat until browned. Add onions, celery, broccoli, and mushrooms. Cover and steam for about 3-5 minutes. Mix the marinade with the cornstarch and pour into the pan. Stir and heat to boiling until sauce thickens. Serve over warm rice.