Preheat the oven to 200 C, 180 Fan, Gas 6 and fill a large pot with water to cook the sprouts, potato and pasta,
When the water boils salt it well and add the prepared sprouts, the diced potato and the pasta and let the water come back to the boil. Cook for 8-10 minutes until the potato is tender and the pasta al dente. Just before draining remove 2 cupfuls of the cooking liquid and put to one side.
Drain the pasta well then tip into a roasting tin or lasagne dish. Add the ricotta and diced Gruyere and a cupful of the cooking water and toss well to combine. Add more liquid if you think it is too dry.
Warm the butter and garlic oil in a small saucepan. When melted and beginning to sizzle gently add the shredded sage and fry for about 30 seconds then spoon the butter and sage over the pasta.
Sprinkle with the parmesan then bake for 20 minutes until scorched to light golden.Leave to stand for at least 15 minutes before serving.