Dice the onion, mince the garlic, peel and slice the carrots, and slice the celery. Add the onion, garlic, carrots, and celery to a 6 quart slow cooker or crock pot.
Remove the skin from the bone-in split chicken breasts, then place them on top of the vegetables in the slow cooker. Add the basil, parsley, thyme, bay leaf, and pepper to the slow cooker.
Pour 8 cups of water over the ingredients in the slow cooker. Place the lid on the slow cooker and cook on high for 4 hours or low for 8 hours.
After cooking on high for 4 hours or low for 8 hours, carefully remove the chicken to a cutting board and discard the bay leaf. Add the egg noodles to the slow cooker, stir to combine, and replace the lid, with the heat turned on to high.
Use two forks to remove the chicken meat from the bones, shredding the meat as you go. Return the meat to the slow cooker and stir to combine with the soup. The noodles should be tender after about 8-10 minutes of being in the soup (this may take a few minutes longer if the slow cooker was previously set to low).
Once the chicken has been returned to the soup and the noodles are tender, begin seasoning the soup with salt. The true flavor of the soup will not be pronounced until it is properly salted. Start with 1 tsp salt, adding more until the soup is bold and flavorful. I use about 1 Tbsp (3 tsp). Finally, stir in a handful of chopped parsley for garnish just before serving.
*I remove the chicken skin from the split chicken breasts before adding it to the slow cooker, but you don't have to. There is a lot of fat and flavor in the skin, so if you want an extra rich soup you can leave it on. You'll just remove the skin at the same time that you debone the chicken meat.
It's not recommended to add frozen meat to slow cookers. It takes too much time for the meat to thaw and come up to a safe temperature, leaving it in the temperature "danger zone" where bacteria can flourish for too long. If you have frozen chicken that you'd like to use, make sure to thaw it fully using a safe method before adding it to the slow cooker.
I suggest using a 6 quart slow cooker. I used a 5 quart slow cooker and it was filled dangerously close to the top.