Preheat your oven to 350°F (175°C).

Melt butter in a large skillet over medium-low heat.

Add diced onion and minced garlic, cooking until the onion is soft, about 3 minutes.

Sprinkle in the flour and cook for 1-2 minutes, stirring constantly.

Lower the heat and gradually add the milk and chicken broth, whisking to avoid lumps until smooth.

Bring the sauce to a boil over medium heat, whisking continuously, and stir in salt and pepper. Allow it to boil for 1 minute.

Grease a 9x13 inch baking dish.

Layer 1/3 of the sliced potatoes in the dish, seasoning them with salt and pepper.

Pour 1/3 of the cream sauce over the potatoes. Repeat the layers two more times, finishing with the cream sauce on top.

Cover the dish with foil and bake for 45 minutes.

Uncover and bake for an additional 35-45 minutes until the top is golden brown and the potatoes are tender.

For a crispy top, broil for 3-4 minutes.

Let the dish rest for 15 minutes before serving.