1. Preheat the oven to 375°F and grease a 9×13-inch baking dish.

2. Peel the potatoes if desired, then slice them thinly and evenly.

3. Melt the butter in a saucepan over medium heat.

4. Add the chopped onion and cook for 3 to 4 minutes until softened.

5. Stir in the garlic and cook for 30 seconds.

6. Sprinkle in the flour and stir for 1 minute to form a roux.

7. Slowly whisk in the milk and chicken broth until smooth.

8. Add the salt and black pepper, then simmer for 3 to 5 minutes until slightly thickened.

9. Layer half of the sliced potatoes in the prepared baking dish.

10. Spoon half of the sauce over the potatoes.

11. Repeat with the remaining potatoes and sauce.

12. Cover the dish with foil and bake for 45 minutes.

13. Remove the foil, sprinkle the cheddar cheese evenly over the top, and bake uncovered for 20 to 25 minutes more.

14. Check that the potatoes are fork tender and the top is golden and bubbly.

15. Let the dish rest for 10 minutes before serving.

16. Garnish with fresh parsley and serve warm.