Slice chicken or tofu thinly and pat dry; season lightly with salt and pepper.
Mix soy sauce, oyster sauce, rice vinegar, honey, and sesame oil in a bowl to make the sauce.
Prepare vegetables: chop broccoli, slice pepper and onion, julienne carrot, trim peas.
Whisk cornstarch into cold water to make a slurry; set aside.
Heat a large wok or skillet over high heat until very hot; add 1 tbsp oil.
Add protein in a single layer; sear without moving until browned, flip and finish cooking; remove to plate.
Add remaining oil to the hot pan; add onions and carrots and stir-fry 1–2 minutes.
Toss in broccoli, peppers, and peas; stir-fry until bright and crisp-tender, about 2–3 minutes.
Push vegetables to the side, add garlic and ginger briefly to bloom their aroma.
Return protein to the pan, pour sauce over, stir to combine, then add cornstarch slurry and cook until sauce thickens and coats everything.
Taste and adjust seasoning, garnish with sesame seeds and green onion, and serve immediately.