- With the rack in the lowest position, preheat the oven to 220 °C (425 °F).
- In a saucepan, melt butter and cook until it just begins to brown. Strain. Set clarified butter aside.
- With a mandoline, thinly slice potatoes: two lengthwise (oblong-shaped) and four widthwise (round-shaped). Thinly slice celeriac. Set aside.
- Generously brush a 23-cm (9-inch) non-stick skillet with clarified butter. Arrange a first layer of round sliced potatoes at the bottom of the skillet, overlapping them to form a rosette.
- Cover the sides of the skillet with all oblong-shaped potatoes slices, placing them perpendicular to those at the bottom of the skillet. Overlap and let them extend over the sides of the skillet.
- Form layers of potato round slices and celery root over the bottom. Brush with a little butter and season every other layer with salt and pepper. Lightly press each layer. Fold potato sides toward the centre. Finish with a potato rosette. Brush with remaining butter.
- Bake for about 1 hour or until the outside of the pancake is golden brown and crispy. Right out of the oven, run a spatula or knife around the edge of the skillet and pancake to slightly loosen it. Flip onto a serving plate. Serve with roasted chicken or beef prime rib.
You can place peeled potatoes in water, but do not soak them once they are sliced, because they will lose their starch and won’t stick together during baking. Make sure you cut potatoes very thinly; you should be able to fold slices without them breaking.