Preheat your oven to 350°F. Grease a 9x13-inch baking dish and set it aside.

In a large saucepan, melt the butter over medium-low heat. Add the diced onion and minced garlic, cooking until the onion softens, about 3-4 minutes.

Sprinkle in the flour and stir for another 2 minutes.

Gradually mix in the milk and chicken broth a little at a time, whisking after each addition until smooth.

Increase the heat to medium and bring to a boil while continuously whisking. Add ½ teaspoon of salt and ⅛ teaspoon of pepper, and let it boil for 1 minute.

In the prepared dish, layer ⅓ of the sliced potatoes and season with ¼ teaspoon of salt and ⅛ teaspoon of pepper. Pour ⅓ of the sauce on top.

Repeat the layering process, ending with the sauce on the final layer of potatoes. Cover the dish tightly with aluminum foil and bake for 45 minutes.

Remove the foil and bake for an additional 35-45 minutes or until the top is golden brown and the potatoes are tender. If you like, broil for 3-4 minutes for extra color.

Let the dish sit for at least 20 minutes before serving to allow flavors to meld.