Combine the sherry, water, and cornstarch in a medium bowl, and stir well to dissolve the cornstarch. Add the beef and stir to season it evenly with the sauce. Set aside for 15 minutes.

In a small bowl, combine the soy sauce, chicken broth, dark soy sauce, if using, salt, sugar, and pepper, and stir to mix well. Place a medium bowl by the stove to hold the bell peppers after their initial cooking.

Heat a wok or large, deep skillet over high heat. Add 1 tablespoon of the oil, and swirl to coat the pan. Add the garlic and ginger and toss well.

Scatter in the bell peppers and toss again. Spread them out into a single layer and cook undisturbed for 15 seconds. Then, cook, tossing often, until they are shiny and just beginning to wilt, about 30 seconds more. Scoop them out into the bowl and set aside.

Let the pan heat up again briefly, and then add the remaining 2 tablespoons oil swirling to coat the pan again.

Add the beef and its marinade, spreading the beef out into a single layer. Cook undisturbed for 30 seconds, and then toss well. Cook, tossing often, until most of the meat is no longer pink, about 1 minute.

Return the bell pepper and any juices in the bowl back to the pan, and toss well.

Add the soy sauce mixture, pouring it in around the edges of the pan. Cook, tossing often, until the peppers are tender but not limp and the beef is cooked through, about 1 minute more. Transfer to a serving plate and serve hot or warm.