Make sure you have all of your ingredients cut and prepped before cooking the curry.
Heat 3 cups of neutral oil in a large wok over medium-high until hot ( ideally 375F). If you do not have a wok, use a large pot (you may need to add more or less oil depending on the size of your pot, you want at least 1 inch of oil in the pot.
Place the pieces of crab into a bowl and toss with 4 tbsp of cornstarch to evenly coat the crab.
When the oil is hot, carefully add the pieces of crab to the oil. BE VERY CAREFUL AT THIS STAGE AS THE OIL MAY SIZZLE AND POP AGGRESSIVELY FOR THE FIRST 30 SECONDS.
Cook the crabs for about 90 seconds on the first side. Adjust the heat on the wok to keep the oil nice and hot. The target temperature for the oil is about 350F after the crabs have been added (the crabs will drop the temperature of the oil)
Flip the crabs over and cook for another 2 minutes, or until the crabs are crispy and lightly golden.
Remove the crabs from the hot oil.
Set the crabs aside on a plate lined with paper towels.
Clean out the wok (or get out your large sautee pan), dry it well, and add 4 tbsp neutral oil. Heat the oil over medium heat.
When the oil is hot, add the onions and stir fry for about 30-45 seconds. They should barely start to soften and get light brown around the edges.
Add the garlic and cook another 15-30 seconds, don't let the garlic burn. Adjust heat as necessary.
Add the curry powder to the wok and stir fry the powder for about 15-30 seconds. It should start to smell aromatic but do not let the powder scorch.
Add the sauce mixture to the wok and allow it to cook over medium heat for about 30-45 seconds. It should boil and reduce as it cooks.
Add the milk and water/stock to the pan and increase the heat to medium high.
Allow the sauce to fully boil and then drizzle in the beaten egg and immediately stir it in well.
Immediately after the eggs have been added to the pan, add the scallion, bird chilies and cilantro.
Immediately after the herbs and chilies, add the fried soft shell crabs. Cook everything together for about 15-30 seconds while stirring so that the sauce is simmering and thickens up slightly from the egg.
Turn off the heat and now adjust the curry. Adjust the consistency of the curry by adding stock or water if necessary in order to achieve a consistency that is thick but still flows nicely. Adjust seasoning with fish sauce and sugar if necessary.
Serve curry with plenty of steamed rice!