Slice the squid open lengthwise and lay flat. Rinse, pat dry, then cut into ½-inch strips. If thick, lightly score the inside in a crisscross pattern.
Whisk together the soy sauce, oyster sauce, Shaoxing wine (if using), sugar, cornstarch, and water. Set aside.
Quickly stir-fry the squid over high heat until it releases liquid. Transfer to a plate and discard the liquid.
In the same pan, heat oil. Cook the ginger until lightly browned, then add the garlic and chili garlic sauce, if using.
Add the onion and cook until slightly softened. Add the bell peppers, chilies, and white parts of the scallions, then cook until tender-crisp.
Return the squid to the pan and toss with the vegetables.
Pour in the sauce and toss until slightly thickened. Add water if needed. Turn off the heat, then add scallions and black pepper.