Extra virgin olive oil, salt, and black pepper, to taste
Wash the vegetables thoroughly. Slice the potatoes into thin slices and the leek into thin strips lengthwise.
In a bowl, beat together the egg with the cream and add a pinch of salt and black pepper. Drain the fish and flake it gently.
Grease the bottom and sides of an oven-proof dish. Prepare the gratin in layers: potatoes, then leek, finely chopped dill, sliced garlic clove, and fish. Cover with the cream and egg mixture. Then repeat the layers as before. Finish off by topping with breadcrumbs, small curls of butter, and black pepper.
Bake in a pre-heated oven at 180°C / 350°F / Gas Mark 4 for around 25 minutes (until golden). When ready, take the gratin out of the oven and wait around 10 minutes before serving.