Step 1

Heat the oven to 200°C/fan180°C/gas Bake the potatoes in their skins for 1½ hours or until soft, leave to cool, then spoon out the flesh into a bowl (see tips). Turn the oven down to 180°C/fan160°C/gas 4.

Step 2

Meanwhile, put the sliced onions and the oil in a large saucepan over a low heat and fry for 45 minutes, stirring often, until golden.

Step 3

Grease a non-stick, loose-bottomed round sandwich tin (about 20cm across) with the butter and line with baking paper. Gently mix the beaten egg and crème fraîche into the potato, then gently mix in the onions and season with salt, pepper and a good grating of nutmeg.

Step 4

Tip the mix into the tin and bake for 45 minutes or until golden.Remove and leave to cool for 5 minutes before turning out. (You may need to run a blunt knife around the edge to release it.) Top with fresh thyme leaves to serve, if you like. http://www.le-gavroche.co.uk/merchandise.html >Click here to buy Michel Roux Jr’s full range of cookbooks.