Preheat oven to 350 degrees.  

In a saucepan over medium-low heat, melt butter, onion, and garlic. Cook until the onion is softened, about 3 minutes. Add flour and cook for 1-2 minutes, stirring frequently.

Reduce heat to low. Combine milk and broth. Add a small amount of liquid at a time to flour/onion mixture, whisking continuously as it thickens to reduce lumps. The mixture will become very thick, continue adding a little bit of liquid at a time, whisking until smooth.

Once all of the liquid has been added, bring to a boil over medium heat while continuing to whisk. Stir in salt and pepper and taste to adjust seasonings. Potatoes can take a generous amount of salt, so add in small increments and keep tasting.

Grease a 9″x13″ baking dish with softened butter. 

Place ⅓ of the potatoes in the bottom and season with salt and pepper. As you layer the potatoes, separate them so they don’t stick together. This way you get sauce in between all the layers. Spread ⅓ of the cream sauce over top. If using Gruyere cheese, sprinkle ⅓ of the amount over each layer.

Repeat layers two more times, ending with cheese.  

Cover with foil and bake for 45 minutes.

Uncover and bake for an additional 45 minutes, or until golden brown and potatoes are tender. Use a knife or fork to poke into the potatoes to ensure they’re soft and cooked through. Allow to rest for 15 minutes before serving.