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 chopped onion and cook until translucent, about 4 minutes.
Add the minced garlic to the skillet and cook for 1 minute until fragrant.
In a large mixing bowl, combine the cooked noodles, sautéed onion and garlic, shredded chicken, thawed peas, cream of mushroom soup, sour cream, shredded cheddar cheese, shredded mozzarella cheese, milk, salt, black pepper, and dried thyme. Stir until well combined.
Transfer the mixture to a greased 9×13 inch baking dish and spread evenly.
In a small bowl, mix the grated Parmesan cheese and breadcrumbs together. Sprinkle this mixture evenly over the casserole.
Bake uncovered in the preheated oven for 30 to 35 minutes, or until the casserole is bubbly and the topping is golden brown.
Remove from the oven and let it cool for 5 minutes before serving.