Preheat the oven to 200°C/fan 180°C/gas mark 6. Put the parsnips, carrots, butternut squash and onion in a large roasting tray and add the garlic and rosemary. Drizzle over the olive oil and season. Stir well to combine, then roast in the oven for 45-50 minutes until tender.
Remove the roasting tray from the oven. Squash the garlic cloves with the back of a spoon and discard the skins. Stir the garlic purée through the vegetables.
Transfer the vegetables to a blender. Add the chicken stock and 50g stilton. Blend until smooth. Divide the soup between four bowls and sprinkle with the remaining crumbled stilton and black pepper. Serve with crusty bread.