Heat the oven to 180C/350F/gas 4. Put the stock in a small pan with the bay leaf, and simmer until reduced by a third. Add the cream and keep warm.
Melt half the butter in a medium saucepan, stir in the flour and continue to cook for one to two minutes, stirring constantly. Reduce the heat to low.
Slowly add the cream mixture, whisking all the time with a balloon whisk, until smooth, thickened and silky. Keep simmering gently for five minutes to cook the flour.
Take off the heat, stir in the nutmeg and half the cheese, and keep stirring until it has melted. Then whisk in the egg yolks and set aside, covering the surface with clingfilm to prevent a skin forming.
Cut the chard leaves off the stems and shred into 1cm-thick ribbons. Cut the stems into 3cm pieces and set both aside separately.
Put the garlic and remaining 25g of butter in a large frying pan, put over a low heat and fry very gently for two minutes, until softened but not coloured.
Stir in the chard stems, cover with a lid or baking sheet and cook for five minutes until the stems are translucent and soft.
Uncover, increase the heat and stir in the leaves, cooking briskly for two more minutes until the leaves wilt and any liquid in the pan has evaporated.
Tip into a large gratin dish and cover with the sauce, tossing to combine slightly. Scatter over the remaining cheese and bake for 25 minutes, until golden and bubbling.