Preheat oven to 180°C. Grease a 2.5L baking dish with melted butter.
Stir the butter, cream, oil, garlic, thyme and bay leaves in a small saucepan over medium-low heat for 2-3 minutes or until melted and smooth. Season with pepper.
Use a mandolin or a sharp knife to cut the potatoes, 1 at a time, into 2mm-thick slices. Discard the end slices of each potato and place in stacks on a work surface. Arrange the slices standing on their sides, 1 stack at a time, in tightly packed rows in the prepared dish.
Strain the butter mixture through a small sieve into a jug. Discard solids. Pour butter mixture evenly over the potato. Sprinkle with salt. Cover with foil and bake for 30 minutes. Remove foil and bake for a further 1 hour or until top is golden and crisp. Set aside for 10 minutes to cool slightly.