Preheat your oven to 180°C (360°F). Prepare your ingredients for assembly.

Peel and slice the onions and garlic. Sauté in half the butter over medium heat until softened and golden, about 15 minutes. Set aside.

Peel and slice the potatoes into thin rounds without washing to retain starch for creaminess.

Grease a baking dish with remaining butter. Layer potato slices and onions, seasoning each layer with salt and black pepper.

Scatter any remaining butter on top. Pour the stock into the dish until it reaches about one-third up the layers.

Cover the dish with aluminum foil and bake for 45 minutes.

Remove the foil and bake for another 30 minutes until the top is golden brown.