Heat olive oil in a large skillet or frying pan over medium-high heat. Season chicken with 1/2 tsp. salt and 1/4 tsp. pepper on both sides. Put the chicken in the heated skillet and brown both sides for about 4-5 minutes per side. Remove the chicken and place on a plate and set aside.
Add the onions to the heated skillet and cook 2-3 minutes until translucent. Add in the garlic, thyme, rosemary, oregano, tomato paste and flour. Cook and stir until combined (about 30 seconds) and the mixture will become thick like a paste.
Stir in the chicken broth and cook for 2 minutes until the gravy has thickened. Be sure to scrape down the bottom pieces stuck at the bottom of the pan. The brown bits have tons of flavor! Turn off the heat.
Add the carrots, celery and potatoes to the bottom of the slow cooker. Season with 1/2 tsp. salt and 1/4 tsp. pepper and stir. Evenly add the onion gravy mixture on top of the vegetables and then place the browned chicken on top (skin side up). Add bay leaves on top of chicken and cover.
Cook on LOW heat for 5-6 hours or HIGH heat for 3-4 hours until the chicken is cooked through and has an internal temperature of 165 degrees F. The vegetables should be tender. Remove the bay leaves, garnish with parsley and serve.