Scalloped Potatoes

¼ cup salted butter
1 large yellow onion diced
2 cloves garlic minced
¼ cup all-purpose flour
2 cups milk
1 cup chicken broth
1 ¼ teaspoon salt divided
½ teaspoon black pepper divided
3 pounds Yukon gold potatoes or red potatoes, see notes, sliced about ⅛-inch thick

Sauce

In a large saucepan, melt the butter over medium-low heat. Add the onion and garlic and cook until the onion begins to soften, about 3 to 4 minutes. Stir in the flour and cook for 2 minutes more.
Combine the milk and broth. Gradually add it to the flour mixture, whisking until smooth after each addition. It will be very thick at first; continue adding a little bit of liquid at a time while whisking.
Once all of the liquid has been added, increase the heat to medium and bring to a boil while whisking. Stir in ½ teaspoon salt and ⅛ teaspoon pepper and let the sauce boil for 1 minute.

Assemble

Layer ⅓ of the potatoes in the prepared dish, seasoning with ¼ teaspoon salt and ⅛ teaspoon pepper. Pour ⅓ of the cream sauce over top.
Repeat the layers, including salt and pepper, ending with sauce. Cover tightly with aluminum foil and bake for 45 minutes.
Uncover and bake for an additional 35-45 minutes or until golden brown and potatoes are tender. Broil for 3-4 minutes if desired.
Allow the potatoes to rest for at least 20 minutes before serving.

Potatoes: Yellow or red potatoes do not need to be peeled. Russets or baking potatoes should be peeled. Russet potatoes may not hold their shape as well as waxy potatoes.
Cheese: Up to 2 cups of shredded cheese can be added to the sauce or between the layers of potatoes. Remove the sauce from the heat after boiling, and whisk in the cheese. Do not boil the cheese, or it may separate. I recommend a bold cheese such as sharp cheddar or gruyere.
Sauce: Herbs such as fresh thyme or minced rosemary can be added if desired. The sauce will thicken as the potatoes rest.