Peel and discard the outer leaves of the lemongrass. With a sharp knife, cut off and discard the upper half of the stalks at the point where the leaves branch out. Thinly slice the remaining stalks.

Process the lemongrass, chile, and garlic in a food processor until finely ground. Transfer the mixture to a mixing bowl. Add the shrimp, 1 tablespoon of the fish sauce, the curry powder, grated ginger, 1/2 teaspoon of the sugar, the black pepper and 1 tablespoon of the peanut oil, mixing together until thoroughly blended. Cover and refrigerate for 30 minutes.

When ready to serve, combine the shrimp stock, coconut milk, remaining 2 tablespoons of fish sauce, remaining 3 teaspoons of sugar and the curry paste. Whisk to blend, then bring to a simmer. Cover and keep warm on a nearby burner.

Heat a wok or large skillet over high heat. Add the remaining 1 tablespoon of peanut oil and heat until smoking-hot. Add onions and red peppers and stir-fry until crisp-tender, 1 to 2 minutes. Transfer to a bowl and set aside. Add another tablespoon of oil and stir fry the mushrooms until slightly wilted and lightly caramelized, 1 to 2 minutes. Set aside with the onions and peppers.

Add the marinated shrimp to the wok, adding more oil if necessary, and stir-fry until the shrimp are lightly golden on both sides, 2 to 3 minutes. Transfer the shrimp to the pot with the coconut-curry broth along with the onions, peppers, and mushrooms and return to a simmer. Stir in the cilantro, scallions, and the juice of 1 lime.  Add the noodles and once they’ve relaxed and softened, 1 to 2 minutes, serve the curry immediately in shallow bowls. Pass lime wedges and a small bottle of fish sauce for guests to season their bowls to their liking.