Preheat the oven to 200C/400F/Gas 6.
In an oiled gratin dish, line the base with a quarter of the potato slices.
Add a quarter of the shredded cabbage to create a layer covering the potato slices. Repeat this layering of sweet potato and cabbage until it is all used up, or the dish is nearly full.
Finally, top with the red onion and drizzle with the oil.
Place into the oven for ten minutes, or until cooked through.
To serve, turn out the gratin onto a plate and top with crumbled Wensleydale.