In a small bowl whisk together oyster sauce, chicken bouillon powder, soy sauce, white pepper, and rice vinegar; set aside.
Prepare vegetables: dice the onion, mince the garlic and ginger, and coarsely chop any large broccoli florets so pieces are bite-sized.
Heat the large nonstick pan over medium-high and add the butter.
While the butter melts, whisk the eggs in a small bowl. Pour the eggs into the pan and swirl to make a thin omelet; cook about 30 seconds until mostly set, folding or breaking into large pieces. Transfer the cooked eggs to a plate and tent with foil.
Add the olive oil to the same pan and increase heat to high. Add the diced onion and cook, stirring, until translucent, about 3–4 minutes.
Stir in the minced garlic and ginger and cook about 20 seconds until fragrant.
Add the frozen peas and carrots, frozen corn, and chopped frozen broccoli; stir and cook 2–3 minutes until thawed and excess water has evaporated.
Add the cold cooked quinoa and the prepared sauce to the pan; cook 1½–2 minutes, stirring, until the sauce evenly coats the ingredients.
Add the cooked egg pieces and sliced green onions, then drizzle the sesame oil over everything. Stir to combine, taste, and adjust seasoning with salt if needed.
Remove from heat and serve hot.