Crumb Topping
Pull meat from chicken and refrigerate until needed.
Add 2 quarts of water to a pot and transfer in chicken bones, skin, and any meat scraps. Add bay leaf and bring to a simmer on high heat. Reduce to low and simmer for 2 hours. Remove to cool, about 1 hour. Strain broth and reserve in refrigerator until needed. This may make more broth than the 6 cups needed.
To make the pot pie mixture, add butter to a pot and melt over medium-high heat. Add onions, carrots, celery, and salt. Cook, stirring, until onions start to turn translucent, about 5 minutes.
Season with pepper, thyme, garlic powder, and cayenne, and cook for 1 minute more. Add flour and cook until flour toasts to a golden brown on the bottom of the pot, 3 to 4 minutes.
Stir in 6 cups of broth, and bring to a simmer on high heat, then reduce to medium. Add peas and cream and cook, stirring, for 10 minutes. Cut chicken meat into small bite-size pieces and add it in. Cook 5 minutes more. Turn off heat and reserve.
Preheat the oven to 375 degrees F (190 degrees C).
Fill a large pot with lightly salted water and bring to a rolling boil. Stir in pasta and return to a boil. Cook pasta uncovered, stirring occasionally, i minute less than specified in package directions.. Drain very well; stir pasta into pot pie mixture. Transfer everything into a large, deep baking dish (about 4 1/2 quart size), and distribute evenly.
Stir panko, salt, and melted butter together in a bowl; sprinkle evenly over casserole.
Bake in the preheated oven until casserole is bubbling around the edges and top is browned, 45 to 60 minutes. Let rest 10 to 15 minutes before serving.