Chinese-Style Beef and Mushroom Stir-fry

I've been cooking beef dishes more and more lately simply because the little girl seems to eat more when beef is on the table.  Not that she eats the beef itself, but she likes the sauce mixed with her rice.  So what I do is mince the cooked beef finely and add them to the rice with the sauce!

500 grams sirloin
1/4 cup oyster sauce
1/4 cup soy sauce
4 to 6 cloves of garlic, minced
1 large onion, sliced thinly
200 grams fresh button mushrooms, sliced
salt and pepper, to taste
cornstarch slurry

Slice the beef as thinly as possible and marinate in the oyster sauce and soy sauce mix for about 20 minutes.

Saute the garlic and onion until soft.  Add the sliced mushrooms.  When the mushrooms are lightly browned, put the beef (and all the marinade) in the pan and stir fry.  I usually add about 1/4 cup of hot water because I want more sauce!  Season to taste and then thicken with cornstarch slurry.  Serve immediately!