Mix the lamb in a baking dish with the other lamb ingredients. Bake at 300 for about 4 hours until tender and shreddy, stirring once or twice during cooking and adding water if needed.
Make the curry paste by adding all the ingredients to a mortar and pestle, starting with the dry ones and mashing them to a powder, then adding each ingredient, in order, one by one, and smashing until mostly smooth. Season with salt
Add some oil to a large pot and saute the curry paste on medium heat for about 7 minutes until fragrant and the raw garlic and onion smell is cooked off.
Add the squash and stir well. Add in the stock and coconut milk and mix, sraping any bits off the bottom that may have collected.
Stir in the turmeric, curry powder, fish sauce, and palm sugar. Add the fibrous stalks from the lemongrass. Bring to a simmer and cook about 30 minutes until the squash is tender.
Remove the lemongrass stalks and blend the soup smooth with a stick blender.
Add the lamb to the soup and all the sauce and juices from the pan along with it.
Meanwhile, bring water to a boil, and heat a thin layer of oil in a frying pan.
Take a handful of the raw noodles and fry them in the oil until crispy, brown, and blistering.
Take the rest of the noodles and boil them in the water for 3 minutes until tender.
Build the soup by first putting some cooked noodles into a bowl, then ladleing in the broth with that tender shredded lamb, and then topping with the crispy noodles and some scallions. Serve immedietly.