Bake the potatoes in their skins in a 200⁰c oven for an hour or more until fully cooked through. Allow to cool.
Peel and slice the shallots and sauté over a low heat in the oil and butter for up to 40 minutes until soft and golden.
Meanwhile, halve the cool potatoes and scoop the flesh out into a bowl, break up with a fork. Add the beaten egg, sour cream and a grating of nutmeg and season. Add the cooked shallots and gently mix.
Grease a cake tin with butter and tip the mixture in.
Bake for 30 minutes or until golden.
Remove and leave to cool slightly before running a knife around the edge and turning out. Dress with a few thyme leaves.