Preheat the oven to 190°C/Gas Mark 5. Peel, halve and deseed the squash. Cut into 2–3cm chunks and put into a roasting tin with the sage leaves, garlic, all but 1 tablespoon of the oil and a generous seasoning of salt and pepper. Roast for about 40 minutes, stirring once, or until soft and coloured at the edges.
Put the remaining 1 tablespoon oil in a frying pan with the butter and place over a medium heat. Throw in the mushrooms along with a little salt and pepper and fry for 4–5 minutes, or until they are cooked through and any liquid they release has evaporated.
For the dressing, in a small bowl, whisk the oil and balsamic vinegar together with some salt and pepper.
In a large bowl, combine the still-warm (but not hot) cooked squash and mushrooms with the rocket and cheese. Add enough dressing to dress them lightly (you may not need it all), toss together and serve.
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