Cut beef in thin slices across the grain. Mix in 1/4 cup soy sauce, 2 tbsp sherry, 2 tsp sesame oil, and 2 tbsp cornstarch, adding the cornstarch last. Marinate for 30 mins.
Remove seeds from the peppers and cut into thin strips, about 2 1/2 inches in length.
In another bowl mix together 1 cup water, 1/4 cup sherry, 1/2 cup soy sauce, and 1/4 cup sugar.
Add 4 tbsp oil to a preheated wok or skillet. Add garlic and ginger and stir-fry briefly until aromatic. Add the beef and stir-fry until it changes color. Remove and set aside.
Wipe the wok or skillet with a paper towel. Add 1 tbsp oil. When the oil is hot, add the red and green peppers. Stir-fry briefly. Add the sauce and bring to a boil. Add the beef. Mix everything through and serve hot over cooked rice.
I like my sauce a little thicker and before adding the beef I usually add a little slurry of cornstarch and water to thicken it. Then add beef.