Preheat your oven to 400 degrees F. Butter a 9x13” baking dish.

Arrange the peeled and sliced potatoes in about 3 layers in the baking dish.

In a 5-quart saucepan, melt the butter over medium heat.

Then add onion and garlic and cook until the onion is soft (about 5 minutes)

Stir in the flour, salt and pepper.

Cook for about 1 minute, until the roux just begins to brown.

Whisk in milk and heavy cream and cook over medium heat until thickened (about 1-2 minutes). Taste and adjust salt and pepper as desired.

Pour the mixture over the potatoes in the baking dish.

Then cover and cook in the preheated oven for 30 minutes.

Remove the foil then continue baking for 30-45 minutes until the potatoes are soft and the top is lightly browned.