Put the water, soy sauce, corn starch, rice vinegar, sesame oil, and chili paste, if using, in a small bowl, mix well and set aside.
Put a large skillet or a wok on the stove, add the oil, and turn the heat to high.
Let the skillet heat for 2 minutes and add the bell pepper, broccoli, and mushrooms and cook until the broccoli is bright green, 2-4 minutes.
Add the garlic and ginger and cook 1 minute.
Add the bok choy (or spinach) and snow peas and cook until the greens are brightly colored but still have some crunch, about 2 minutes.
Add the sauce and bring to a boil.
Add the scallions and bean sprouts and cook 1 minute. Serve right away.