1. Pre-heat the oven to 180°C
2. Peel and slice the potatoes into thin rounds. Place into a large pan of salted water and boil for about 8 mins until tender but not completely cooked through. Drain and set aside.
3. Drain the pilchards from the sauce and set aside.
4. Gently fry the onion in a pan with a little oil until soft. Add the butter and once melted add the flour then cook for 1-2 minutes.
5. Remove from the heat and gradually whisk in the milk until smooth. Bring to a boil, stirring to avoid any lumps and sticking at the bottom of the pan. Then simmer for 3-4 minutes, add ¾ of grated cheese until melted and the sauce thickens.
6. Flake the pilchards into the sauce, add the peas and adjust seasoning with salt and pepper.
7. Place the pilchard cheese sauce into a casserole dish and top with the semi cooked potato slices. Drizzle with little oil and top with the rest of the cheese.
8. Bake for 20 minutes until lightly browned and bubbling.