Prepare the cabbage : Sprinkle the chopped cabbage with a small amount of salt and massage gently to combine. Let it sit for 5–10 minutes, then squeeze out the liquid (but don’t over-squeeze) by using your hands or a clean cloth. Set aside.
Prepare the filling : In a bowl, mix the minced pork with salt, ginger, spring onion whites, white pepper, soy sauce, oyster sauce, sesame oil, and chicken stock. Stir in one direction until the mixture becomes sticky and well combined.
Combine and chill : Add the squeezed cabbage to the pork mixture and mix until evenly incorporated. Cover with plastic wrap and refrigerate for at least 2 hours to allow the flavours to meld.
Wrap the dumplings : Place a spoonful of filling in the centre of a dumpling wrapper. Lightly moisten the edges with water, then fold the wrapper in half to form a half-moon and press the centre to seal. Starting from the centre, make small pleats along one side of the wrapper toward one end, then repeat on the other side until fully sealed, keeping the back edge flat. Press firmly along the edge to secure the crescent shape. Repeat with the remaining wrappers and filling.
Prepare the sauce : In an oven-safe roasting pan, stir together the gochujang, tomato purée, minced garlic, soy sauce, honey, sesame oil, dashi, and milk until smooth and well combined.
Assemble : Add the coriander, sliced courgettes, and dumplings to the pan, arranging the dumplings so their tops are slightly exposed.
Bake (covered) : Cover the tray with foil and transfer to a preheated oven at 190°C. Bake for 20 minutes, or until the dumplings are cooked through and the sauce is bubbling.
Finish baking (uncovered) : Remove the foil and continue baking for a further 10 minutes, until the sauce thickens and becomes creamy.
Serve : Sprinkle with additional coriander and sesame seeds. Spoon the sauce generously over the dumplings and enjoy!