Lightly spray the inside of your crockpot with nonstick cooking spray.
Add the frozen diced potatoes, condensed cream of celery soup, diced onion, sour cream, 4 tablespoons of melted butter, garlic, salt, and black pepper.
Add 1 cup of freshly shredded cheddar cheese. Reserve the remaining 1 cup for the topping.
Stir everything together until the potatoes are evenly coated in the creamy mixture.
Cover and cook on HIGH for approximately 3 hours, stirring once every hour, until the potatoes are tender.
Once the potatoes are cooked, remove the lid and sprinkle the remaining 1 cup of freshly shredded cheddar cheese evenly over the top.
Replace the lid and leave the crockpot on HIGH until the cheese melts, approximately 1–5 minutes, depending on your slow cooker.
Meanwhile, measure 1 cup of French-fried onions and gently crush them into smaller pieces.
Melt 2 tablespoons of butter in a skillet over medium heat.
Add the crushed French-fried onions and gently stir for approximately 3–5 minutes, until lightly toasted and golden. Watch carefully so they don’t burn.
Once the cheddar cheese has melted over the potatoes, sprinkle the buttery, crispy French-fried onions evenly over the top.
Serve immediately while warm and enjoy!