Preheat oven to 350 F degrees.
Heat a large oven safe pot (like cast iron) over MED heat. Add 1/2 cup butter and melt. Add mushrooms and onion and cook, stirring occasionally, until tender, about 5-6 minutes. Add garlic and cook an additional minute.
Stir in thyme, salt, and pepper, then add flour, stirring to coat completely.
Add in chicken stock and heavy cream, stirring or whisking frequently to remove any flour lumps. Cook, stirring often, about 8-10 minutes, until thickened.
Stir in prepared egg noodles, shredded chicken, frozen peas, and Parmesan cheese.
In a small mixing bowl, combine remaining 1 Tbsp melted butter with panko breadcrumbs, then sprinkle over top of the dish. Season with additional black pepper if desired, then bake, uncovered, for 35-45 minutes until golden brown and bubbly.