1. Preheat the oven to 180C/gas 4. 2. Heat 2 tablespoons of the olive oil in a frying pan. Cook the onion and garlic on a medium heat until softened, about 3 minutes. Set aside. 3. Slice the potatoes no thicker than a pound coin. Heat enough oil to cover the bottom of a large frying pan. Fry the potato slices until coloured on both sides, about 4 minutes. Season with a little salt and freshly ground black pepper. Remove from the pan and place on kitchen paper to blot away excess oil. 4. Generously butter a ceramic oven dish and layer the potatoes, slices of Marlin, onion, and parsley, seasoning each layer with salt and pepper. Squeeze a splash of lemon juice and add a sprinkling of zest as you build up the layers. 5. Dot the top layer with a few knobs of butter and pour over enough stock to cover. Finally, sprinkle over the crushed fennel seeds and bake for about an hour, or until the potatoes are soft enough to crush between your fingers. Serve with a hunk of fresh bread.