Make the stock: Adjust the racks in the oven so a large pot can fit in the middle and heat to 200°F. Place the turkey, lemongrass, garlic, shallot, and sugar in the large pot and top with water. Bring to a gentle simmer, then very carefully transfer to the oven, uncovered, and cook in the oven for 2 hours.
Meanwhile, make the chili oil: in a small, deep sauce pan, heat the oil and chili peppers over medium low heat, stirring often, until it smells aromatic. Remove from the heat and strain with a mesh strainer. Add the oil back to the pan along with the shallots, garlic, and lemongrass. Cook over medium heat, stirring, until the shallots just begin to soften, about 2-3 minutes. Remove from the heat and stir in rock sugar until dissolved, then stir in the red pepper flakes, fish sauce, and shrimp paste.
Remove the stock from the oven and carefully strain into a large pot. Season it with fish sauce, by the tablespoon until it is aggressively seasoned but not overly salty, about 3-4 tablespoons. Keep the stock on a low boil.
Assemble: Cook the noodles according to the package. Drain well and place the noodles in large warmed bowls and top with about 2 cups of the spicy lemongrass broth, 1-2 tablespoons of chili oil (to taste), and leftover turkey. Add sliced red onions, mint, cilantro, green onions, and lime to taste. Add more chili oil if you like it hot! You will definitely end up with more chili oil than stock so you can save it for another round of whatever soup you might have on hand. Enjoy :)