Heat butter and olive oil in a large skillet over medium heat. Add onions, celery, carrots, 3/4 tsp. salt and 1/2 tsp. pepper and cook, stirring until onions and celery are golden. Stir in flour and cook an additional 2 minutes. Transfer to slow cooker.
Stir in stock, chicken, noodles and thyme. Cook on low 6-8 hours. Season with salt and pepper to taste.