Heat oven to 190C/170C fan/gas 5 and butter a 2L gratin dish.
Peel the potatoes and slice into rounds as thinly as possible – use a mandolin if possible. Next, remove the stem from the sorrel or spinach leaves, roll up into a cigar shape and cut into thin strips. Thinly slice the onions and separate them with your fingers.
Melt the butter in a large pan over a low heat and add the sorrel or spinach, sprinkle with a little sea salt and cook until it begins to wilt. Pour in the cream and milk and bring to a gentle simmer for a minute, remove from the heat.
Put the potatoes and onions in a saucepan with a teaspoon of sea salt and cover with water. Bring to the boil and cook for about 10 minutes, shake the pan from time to time so the potatoes don’t stick to the bottom. Remove from the heat and drain well.
Make a layer of potato and onion in the bottom of the gratin dish and spoon over 1/3 of the cream mixture. Add the remaining potatoes and onion and finish with the rest of the cream mixture, sprinkle with a fine layer of nutmeg. Finish with the breadcrumbs and herbs sealing the potato layers and bake for 50-60 minutes until golden on top and soft through.
