Preheat the oven to 425°F (220°C).
Brush a large rimmed baking sheet with 1 tablespoon of vegetable oil to ensure even coating.
Place the frozen dumplings on the baking sheet in a single layer, flat side down, with at least 1 inch of space between each dumpling.
In a small bowl, whisk together the water, cornstarch, flour, and the remaining 1 tablespoon of vegetable oil until the powders are fully dissolved.
Carefully pour the starch slurry onto the baking sheet around the base of the dumplings.
Transfer the baking sheet to the oven and bake for 18 to 20 minutes, or until the liquid has completely evaporated and a golden-brown, lacy crust has formed.
Remove from the oven and allow the dumplings to rest for 2 minutes on the pan to further crisp the skirt.
While resting, combine the soy sauce and sesame oil in a small dipping bowl.
Use a wide spatula to carefully lift the dumplings and their connected crust from the pan; garnish with scallions and sesame seeds before serving.