Combine all the ingredients except potatoes and the extra Manchego cheese in a large bowl. Add the potato slices and mix well.
Press the potatoes in a large lightly oiled baking dish, taking care with the top layer to ensure it is flat, compressed and almost covered with the cream mixture. Cover the tray with non-stick cooking paper then aluminium foil and seal the edges as much as possible.
In a preheated 170°C/338°F oven, bake for 1 hour 20 minutes, then remove the paper and foil and scatter the remaining cheese on top.
Cook, uncovered, at 160°C/320°F for a further 30 minutes or until the cheese is nicely golden and potatoes are easily pierced with a skewer. Rest for 5 minutes then score the top into portions. Serve immediately.