Preheat your oven to 375 degrees. Lightly grease a baking dish and set aside.
Warm the milk in a small saucepan over medium-low heat.
Lay 1/3 of the potato slices over the bottom of the prepared baking dish, followed by 1/3 of the leeks and a sprinkle of salt and pepper. Sprinkle 3 tbsp of flour over the potatoes and top with 2 tbsp of butter, cut into small pieces. Repeat two more times so you end up with three layers, totoal.
Pour the warm milk over the potatoes until it just reaches the top layer. Bake for 45-60 minutes, or until the milk has thickened into a creamy sauce.
Let the potatoes set for 10 minutes, then serve warm.
Notes
Recommended low FODMAP serving – 1/6 of total recipe