To make the sauce, melt butter in a large saucepan. Add 3 coves of garlic and cook, stirring frequently, until fragrant, about 1 minute. Whisk in flour until lightly browned, about 1-2 minutes. Gradually whisk in chicken stock, 1/2 thyme, dried parsley and onion powder. Cook, whisking constantly, until the sauce has begun to thicken, about 5 minutes; season with 1 tbsp salt and 1 tsp pepper, to taste. Set aside.
In a large pot of boiling salted water, cook egg noodles according to package instructions; drain well.
Heat 1 tablespoon olive oil in a large stockpot or Dutch oven over medium heat. Season chicken thighs with 1 tbsp salt and 1 tsp pepper, to taste. Add chicken to the stockpot and cook until golden, about 2-3 minutes; set aside.
Add remaining 1 tablespoon oil to the stockpot. Add 3 cloves of garlic, onion, carrots and celery. Cook, stirring occasionally, until tender, about 3-4 minutes. Stir in thyme until fragrant, about 1 minute.
Stir in egg noodles, sauce and Parmesan until well combined and heated through, about 3-4 minutes.
Serve immediately, garnished with parsley, if desired.