Cut the squash in half lengthways and carefully scoop out the seeds. Season the shells lightly with salt and pepper and place, cut side up, in a roasting tin. Drizzle with a little of the oil and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 45 minutes.
Dry-fry the pancetta for about 5 minutes, until it is golden and has released its fat. Lower the heat, add the remaining oil and gently fry the garlic and sage for a further 4–5 minutes, until the garlic is softened.
Remove the squash from the oven and fill the hollows with the pancetta mixture. Spoon in the mascarpone and scatter over the sun-dried tomatoes and Parmesan. Return to the oven for 15–20 minutes, until bubbling and golden. Garnish with sage leaves and serve with some grilled bread and a crisp green salad.