Preheat oven to 375°F (190°C). Grease bottom of a 8 x 12-in. baking dish with 1 tbsp (15 mL) of oil. Starting at the short side of the baking dish, layer potato slices upright, slice after slice, until potatoes fill the dish. Slide shallot slices randomly between potato slices, distributing as evenly as possible. Brush tops of potatoes with remaining oil and sprinkle with salt and pepper. Bake uncovered, on middle rack of oven, 1 hour.
Reduce oven to 300°F (150°C). Sprinkle thyme leaves over potatoes and cook until tops are crispy and potatoes are tender, about 30 min.
The key to this dish is very thinly sliced potatoes – use a mandolin or sharp knife to achieve this result.