Melt butter in a large sauté pan over medium heat. Add the ground turkey and cook until the meat is no longer pink.
Stir in onion, celery, carrot, and garlic and cook for 5 minutes.
Add flour and stir to coat meat and vegetables. Cook for 1 minute, then add in milk. Stir the mixture until smooth.
Bring mixture to a boil and cook for 2 minutes. Transfer the mixture to a 4-qt crockpot.
Stir in chicken stock and potatoes. Cover and cook on low for 4-5 hours until potatoes are tender.
Add Cheddar cheese to soup and stir until cheese melts. Season with salt and pepper to taste.
Garnish servings with ketchup and mustard.
Notes
Dice the potatoes into small, equal-sized, pieces to insure even cooking time.
Check the potatoes for tenderness before adding the Cheddar cheese. Depending on the size of your potato pieces, you may need additional cooking time.
Wait to add the cheese until just before serving. The cheese will curdle if you add it while the soup is still cooking.
If you don’t have a crockpot or slow cooker, you can make this cheeseburger soup in an Instant Pot. I used an 8-qt Instant Pot on the slow cooker setting.