Blanch bok choy in boiling water for 1–2 minutes and immediately place in an ice bath to stop cooking. Set aside.
Season flounder with salt and pepper.
Place fish in steamer and steam for 8–10 minutes, until flesh is flaky and tender.
While fish is steaming, heat canola oil in a hot pan and add onion, 1 teaspoon garlic, and peppercorns. Add salt and pepper.
When onion has softened, add black vinegar and chili oil. Set aside.
In a separate pan, reheat bok choy and add remaining garlic and stock or wine.
To serve, place bok choy on bottom of a serving tray. Place steamed flounder on top, and pour onion mixture and sauce over flounder. Garnish with chopped leeks and citrus segments and drizzle with sesame oil.