Preheat the oven to 180°C.
Drizzle butternut squash with one tablespoon of oil and bake for 8-10 minutes until tender.
Fry sliced onion and garlic in the remaining oil for 4 minutes until soft.
Add leeks to the pan and cook for another 4 minutes.
Mix in the roasted squash and wine, then cook for 6 minutes before seasoning.
Stir in the crème fraîche, double cream, chickpeas, and tarragon, then transfer to a casserole dish.
Increase oven temperature to 200°C.
Boil potatoes in salted water for 12 minutes until tender, then drain and mash.
Mix flour and butter into the mashed potatoes to form a dough.
Roll dough to 2.5cm thickness and cut 10cm circles to place over the casserole.
Bake for 12 minutes until the top is golden-brown.