Heat oil in a large enameled cast iron skillet over medium-high heat. Pat chicken thighs dry and season with salt and pepper. Brown chicken thighs skin side down until dark golden brown. Once browned transfer chicken thighs to slow cooker.
Reduce pan heat to medium. Remove any excess fat from pan, leaving 2 tablespoons behind. Add onions, carrots, and celery and sauté or about 5 minutes. Add garlic and sauté for another minute. Add tomato paste and sauté for another minute. Transfer sautéed vegetables and any pan drippings to slow cooker.
Add water, chicken bullion, and fresh thyme to slow cooker. Cook on low for about 5 hours or high for about 3 1/2 hours. Once carrots and celery are fork tender, remove chicken thighs from slow cooker and set aside to cool. Once cooled, remove skin and bones and return shredded chicken to slow cooker. Remove and discard thyme stems. Meanwhile, prepare egg noodles according to package instructions. Once egg noodles are cooked add to slow cooker. Stir and season with salt and pepper as needed. Stir in apple cider vinegar and add fresh parsley.