So, while cooking the meat or veggies, mix the following into a glass or measuring cup:
1/4-1/3 cup cold water
1-2 Tbsp cornstarch
2 Tbsp soy sauce
2 Tbsp Xiao Xing (sub sherry, sake or dry white wine)
1+ Tbsp oyster sauce
1 Tbsp hoisin sauce (optional)
1 Tsp rice wine vinegar
1Tsp sugar (white or preferably brown)
1 Tsp sesame oil
1+ Tbsp sambal (optional for spiciness)
Be sure to stir before you dump it in. While you can just dump it straight into the stir fry, I prefer to remove the stir fried meat and veggies to the serving dish, add a touch of oil, sautée minced garlic and ginger in wok for 45 seconds, then dump this sauce in, give it 15 or so seconds (at high heat) on its own, stirring, then dump meat/veggie combo back into wok and stir it all together. This helps the sauce coagulate better, and after about 1 minute it is nice and thick and ready to serve.