In a large bowl, mix together the onion, green pepper, mushroom soup, milk, mozzarella cheese, cheese spread, celery seed, paprika, and parsley until thoroughly combined. Pour 1/4 of the sauce into the bottom of a slow cooker. Spread the potatoes over the sauce; pour the remaining sauce over the potatoes. Place chicken breasts into the slow cooker, sprinkle with a pinch or 2 of sea salt, and push the chicken down into the sauce mixture.
Cook on low for 6 hours, then set cooker to high and cook until chicken and potatoes are very tender, 4 additional hours. During the last hour of cooking, mix cornstarch with water, and stir into the sauce to thicken.