Preheat the oven to 350°F (175°C).
Cook the egg noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, melt the butter over medium heat. Add the diced onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet. Cook for another 5 minutes until mushrooms are tender.
In a large mixing bowl, combine the cooked noodles, cooked chicken, sautéed onion, garlic, and mushrooms, frozen peas, shredded cheddar cheese, cream of mushroom soup, sour cream, milk, dried thyme, salt, and black pepper. Stir until all ingredients are well combined.
Transfer the mixture to a 9×13 inch baking dish and spread evenly.
Sprinkle the bread crumbs evenly over the top of the casserole.
Bake uncovered in the preheated oven for 35-40 minutes until the casserole is bubbly and the bread crumbs are golden brown.
Remove from oven and let cool for 5 minutes before serving.