Wash the on choy thoroughly to remove grit. Separate tender leaves from thicker hollow stems. Cut stems into 2–3 inch lengths; leave leaves whole or roughly chop if very large.

Prepare tofu

Pat tofu cubes dry on paper towel to remove excess moisture so they hold texture in the sauce.

Make bean curd paste and slurry

Mash the fermented bean curd into a smooth paste. In a small bowl combine the mashed bean curd, soy sauce, oyster sauce, and stock; stir to dissolve. In a separate small cup mix cornstarch with about 1 tablespoon cold water to make a slurry; set aside.

Cook Tofu

Pan-fry tofu

Heat 1 tablespoon oil in a large skillet or wok over medium-high heat. When hot, add tofu cubes in a single layer and fry, turning occasionally, until golden on at least two sides, about 4–6 minutes. Remove tofu to a plate and set aside.

Cook Vegetables & Build Sauce

Sauté aromatics

Wipe the pan if necessary, then add remaining 1 tablespoon oil and return to medium-high heat. Add ginger, the white parts of scallion, and minced garlic. Stir-fry until fragrant, about 20–30 seconds, taking care not to burn the garlic.

Stir-fry stems first

Add the on choy stems to the pan and stir-fry for 1–2 minutes until they begin to soften and turn translucent but still have a slight crunch.

Add sauce and simmer

Pour the prepared bean curd–stock mixture into the pan. Bring to a gentle simmer, scraping up any browned bits. Stir in sugar, white pepper, and salt to taste. Let the sauce reduce slightly for 1–2 minutes so flavors marry.

Return tofu and thicken

Add the fried tofu and on choy leaves to the pan. Stir gently to combine. Give the cornstarch slurry a quick stir (slurry may settle) and pour it slowly into the simmering sauce while stirring; the sauce should thicken in 30–60 seconds to a glossy consistency that coats the vegetables and tofu.

Finish and season

Stir in toasted sesame oil and the green parts of the scallions. Taste and adjust seasoning — add more salt, sugar, or a dash of soy sauce if needed. If using chili, add now.

Serve

Transfer the on choy with bean curd sauce to a serving plate or shallow bowl. Sprinkle with toasted sesame seeds if using. Serve immediately with steamed rice.