Bring a large saucepan of salted water to the boil. Cook the pasta according to packet instructions, then drain and set aside.

Heat the oil in a large frying pan over medium-high heat. Toast the sage leaves until crisp and translucent, then remove with a slotted spoon and set aside.

Melt the butter in the pan. Stir-fry the mushrooms and garlic for 4 minutes until golden. Sprinkle over the flour and cook for 1 minute, then gradually stir in the stock and milk. Cook for 5 minutes or until thickened. Season to taste with salt and pepper. Add the pasta and toss to combine.

Divide the pasta among bowls. Spoon over the ricotta, scatter with the walnuts and sage and drizzle with oil, to serve.