Put cold legs and wings into a stock pot just large enough to fit in a single layer. Add enough cold stock to cover and bring to a bare simmer over medium heat (do not boil). Skim off any cloudy particles that float to the surface. Reduce heat and poach (without boiling) 5-6 hours or until meat separates easily from the bones when a meat fork is inserted. Transfer meat to a bowl and cover loosely with aluminum foil until cool enough to handle. Pull all meat off the bones and dice into bite-size pieces; cover and set aside. Pour stock through a fine strainer and return to the pot; there should be about 5 cups (if not, add more stock to equal 5 cups).

 Heat 2 Tbsp. butter in a sauté pan over medium heat. Add the leeks, asparagus, and a pinch of salt and sauté until leeks have lost their sharpness and asparagus is tender, about 10 minutes; set aside. Cook egg noodles according to package instructions, drain, and toss with 1 Tbsp. butter to coat and separate the noodles; cover and set aside.

 Bring stock to a simmer over medium heat. In a small bowl whisk together the heavy cream and flour to make a slurry. Stirring constantly, add the slurry to the stock and continue to cook and stir another 3-4 minutes until mixture thickens slightly. Reduce heat to low and whisk in 2 Tbsp. of potato flakes. Wait a few minutes to allow the flakes to completely hydrate then add more, 1 Tbsp. at a time until desired thickness is achieved. Gently stir in the meat, sauteed vegetables, and the remaining tablespoon of butter and heat through.

 To serve, divide the egg noodles between four large soup bowls, top with spinach and ladle the hot soup over top.