Melt the butter in a large pan over medium heat. Add the diced onion and cook for 5 minutes until softened.
Add the diced chicken and cook for 5-6 minutes until browned on all sides.
Stir in the garlic and plain flour, cooking for 1 minute.
Gradually pour in the chicken stock and milk, stirring constantly to avoid lumps.
Add the cubed potatoes, sweetcorn and dried thyme. Bring to a simmer.
Cover and cook for 15 minutes until the potatoes are tender and the soup has thickened.
Season with salt and pepper to taste. Ladle into bowls and serve with crusty bread if you have it.