Preheat the oven to 180°C/gas 4. Brush the 2 tablespoons of olive oil over a suitable baking dish (I used one 1.5 litres in capacity) and arrange half the potato slices on the bottom of the dish. Scatter half the feta and sage on top, sprinkle with a little salt, then lay the celeriac slices on top of that, followed by the leek and the remaining feta and sage. Lay the remaining potato slices on top, pour in the 150ml boiling water, then lightly season (remembering the feta will be a little salty). Brush one side of a sheet of non-stick baking parchment the same size as the dish with a little extra olive oil and lay this side on top of the potatoes. Cover with foil and seal tightly.
Bake for 1½ hours, then remove the foil and baking parchment, and place the dish under a hot grill to colour the potatoes. Serve from the dish while piping hot.