1. Stir together ginger, soy sauce, vinegar, and sesame oil in a bowl; set aside.

2. Heat 1 1/2 teaspoons vegetable oil in a wok or large skillet over medium-high heat until hot but not smoking. Add celery and garlic; cook, stirring constantly, until just tender, about 4 minutes. Transfer to a rimmed baking sheet; set aside.

3. Add 1 1/2 teaspoons vegetable oil to wok. Add snow peas and mushrooms; cook until tender, about 4 minutes. Transfer to the baking sheet; set aside.

4. Reduce heat to medium-low; add 1 1/2 teaspoons vegetable oil to wok. Add bok choy; cook, turning frequently, until tender, 6 to 8 minutes. Transfer to the baking sheet; set aside.

5. Raise heat to medium-high. Add remaining 1 1/2 teaspoons vegetable oil to wok. Add bell pepper, and cook until tender, about 2 minutes. Add reserved soy-sauce mixture, and cook until reduced by half, about 2 minutes. Add reserved cooked vegetables to wok, and cook, tossing frequently, until vegetables are heated through, about 3 minutes.

6. Remove wok from heat; toss in scallions and pea shoots. Season with salt and pepper. Sprinkle with sesame seeds, and with red-pepper flakes, if desired.