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Preheat the oven to 425 degrees F.
Heat the oil in an oven-safe braiser over medium heat and add the onion, celery, white parts of the scallions, and a pinch of salt.
Cook, stirring occasionally, until softened, 7 to 10 minutes.
Add the ginger, garlic, and crushed red pepper and cook for another minute, until fragrant.
Whisk in the flour and cook, whisking, for 2 more minutes.
Add the chicken stock very gradually, in 3 or 4 additions, whisking continuously and allowing the mixture to thicken before each addition.
Stir in a handful of the scallion greens (reserve the rest for the topping) and the peas, then reduce the heat to low.
Season the chicken with salt if necessary and then add it to the braiser.
Stir in the heavy cream, freshly ground black pepper, vinegar, and soy sauce.
Taste and adjust the seasoning as desired.
Roll out two sheets of puff pastry and press together at the seams to make one long puff pastry. The sheet should be 18 inches across by 12 inches wide.
Brush the surface with sesame oil.
Sprinkle with the green onion tops and a pinch of salt.
Roll up like a snake, then spiral like a snail shell.
Use a rolling pin to flatten and roll out to 2 inches larger than your cooking vessel.
Slice 4 steam vents.
Egg wash the edges of the vessel.
Place the pancake on top of the vessel, then brush with egg wash and sprinkle with sesame seeds and flaky salt.
Put in the oven with a pan underneath to catch drips.
Bake for 30 minutes, tenting with foil the last 10 minutes.
Serve with Chinese hot mustard if desired.