Spray a skillet with cooking spray and heat over medium heat. Add chicken to skillet and cook, stirring frequently, about 4 minutes. Add onion and cook until chicken is browned and onion is slightly translucent, about 3 more minutes. Transfer to the slow cooker.

Add carrots, celery, bell pepper, thyme, and pepper to the slow cooker; mix to combine. Stir in diced tomatoes and enough chicken broth so vegetables are just covered with liquid.

Cover and cook on Low for 7 hours.

While the soup is cooking, fill a large pot with lightly salted water and bring to a rolling boil. Stir in egg noodles and return to a boil. Cook pasta uncovered, stirring occasionally, until tender yet firm to the bite, 6 to 8 minutes. Drain and set aside.