Preheat the oven to 220 C / Gas 7. Place the rack in the centre of the oven.
Mix the squash and onion together in a roasting tin. Mix in the grapes and season with salt and pepper.
Melt the butter, then whisk in the olive oil and the herbs. Pour the buttery mixture over the squash, onion and grapes and toss to coat thoroughly.
Roast in the oven for about 45 minutes, turning once at 20 minutes. Check for doneness. The squash should be slightly crisp on the surface.
While the squash is roasting, toast and chop the pine nuts. Remove the squash from the oven, transfer it to a serving dish and sprinkle the pine nuts and crumbled feta over it. Serve straightaway or at room temperature.