Preheat the oven to 180°C/350°F/Gas mark 4. To make the pumpkin topping, work the cold butter into sieved flour. Rub with your finger until it forms a sandy texture.
Add the salt, cheese and nuts.
Cut the vegetables into small, bite-size pieces and sweat in a large pan. Stir occasionally, until the vegetables start to take on colour. Add the onion and garlic and sweat under foil. Pour into an ovenproof dish and sprinkle with the topping. Bake for 20 minutes.