Preheat the oven to 350°F (175°C).
In a large skillet, heat olive oil over medium heat and add sliced onions.
Cook onions for about 15-20 minutes, stirring occasionally, until golden brown and soft.
In a mixing bowl, combine the cooked chicken, egg noodles, caramelized onions, chicken broth, sour cream, dried thyme, salt, and pepper.
Transfer the mixture into a greased 9x13 inch casserole dish and spread it evenly.
Top the mixture with shredded Gruyère cheese.
Bake in the preheated oven for 25-30 minutes, until bubbly and golden.
Remove from the oven and let it cool for about 5 minutes before serving.