1 (10.75 ounce) can condensed cream of mushroom soup
22 fluid ounces milk
Combine mozzarella cheese, eggs, cracker crumbs, pecans, onion soup mix, and celery salt in a large bowl. Form mixture into small meatballs.
Heat oil in a deep-fryer or large saucepan. Cook meatballs in batches until browned and crispy, about 5 minutes. Drain on a baking sheet lined paper towels.
Transfer meatballs to a large slow cooker. Cover with cream of mushroom soup. Use the empty can to measure and pour in milk. Cook on Low until flavors combine and soup mixture thickens, 30 minutes to 2 hours.