Place a rack in centre of oven. Preheat oven to 220°C (200°C fan-forced). Place a heavy baking tray in oven.
Place pumpkin, cut-side down, on hot tray. Drizzle with oil to coat. Roast for 40-45 mins or until a knife can easily pierce through softened pumpkin. Set aside to cool slightly. Use a spoon to remove pumpkin flesh and place in a blender. Discard skin. Reduce oven temperature to 90°C (70°C fan-forced).
In a small saucepan, bring 1 cup (250ml) of cream to a simmer. Pour into blender. Rinse and dry saucepan thoroughly. Carefully blend warm cream, pumpkin and ¼ tsp of salt until smooth.
In the clean saucepan, stir ½ cup (110g) of sugar and 2 tbs of water over medium-high heat, occasionally brushing sides of pan with a wet pastry brush to remove sugar crystals, until sugar dissolves. Bring to the boil, without stirring. Cook, brushing down side of pan and swirling occasionally to ensure caramel cooks evenly, for 6 mins or until caramel is dark golden brown. Remove from heat. Whisk in pumpkin mixture. Cover to keep warm.
Whisk milk, remaining 1 cup (220g) of sugar, eggs and spices in a baking dish until smooth. Working in batches, soak bread in milk mixture, turning once, for 4 mins or until mixture is absorbed.
Heat 2 large heavy non-stick frying pans over medium heat. Melt 20g of butter in each pan. Place 3 slices soaked bread in each pan. Cook for 2 mins each side or until bread is golden brown and heated through. Place French toast on baking tray in oven to keep warm. Wipe pans. Repeat to cook remaining bread.
In a bowl, whip remaining ²⁄³ cup (160ml) of cream to soft peaks. Divide French toast among 6 plates. Spoon over pumpkin caramel and whipped cream. Serve sprinkled with walnuts.