Preheat oven to 180°C/350°F/Gas Mark 4. Peel or scubr potatoes and slice 1 cm(½ in) thick. Put in a deep saucepan and cover with cold salted water. Barely bring to the boil and drain.
Meanwhile, heat butter up in a deepish saucepan and add onions. Cook over a moderate heat until wilted but not coloured. Add herbs, seasoning and mustard cook 1 minute. Add cream and bring to the boil.
Rub remaining butter over the base of a deepish 30 cm(12 in) square oven dish. Put in half the potatoes, pour over half the onion mixture, add remaining potatoes, then remaining onions. Seal with foil. Bake 40 minutes. Remove foil and cook until golden and potatoes are tender. Eat hot or save for the next day and it’ll taste even better when re-heated.