In a small bowl, whisk together ⅓ cup soy sauce, 2 tablespoons hoisin, 1½ tablespoons grated ginger, and 1 teaspoon sriracha. Set aside.
Bring a large pot of water to a boil and season generously with salt. Fill a large bowl with ice water. Add 4 cups chopped bok choy to boiling water and cook just until tender, about 2 minutes. Use a spider or slotted spoon to transfer bok choy to ice bath. Add 1½ cups snow peas to boiling water and cook 1 minute, transfer to ice bath. (Reserve boiling water.) Once cool, drain vegetables.
Add 12–14 ounces lo mein noodles to boiling water and cook 1 minute less than package directions. Drain and toss noodles with 1 tablespoon sesame oil. Set aside.
In a large sauté pan or wok, heat 1 tablespoon sesame oil over medium. Add 1 pound shrimp and cook just until pink, about 1 minute per side.
Add ¼ teaspoon pepper flakes and 3 cloves grated garlic and stir-fry until fragrant, about 30 seconds longer. Transfer shrimp to a plate.
Heat remaining tablespoon sesame oil over medium. Add bell pepper and drained bok choy and snow peas, sauté 2 minutes. Add cooked noodles and sauce and gently toss until everything is evenly coated, about 2 minutes.
Add cooked shrimp and scallions; garnish with sesame seeds and serve hot.