Heat the oil in a large, heavy-bottomed saucepan over a medium heat, then add the onions, garlic, carrot and celery and sauté for a couple of minutes. Next, add the chopped pumpkin, sage, chilli, sweet potato and butter beans and continue to cook for a further 2 minutes.
Pour in the vegetable stock, stir and bring to the boil, then turn down the heat to a gentle simmer. Cover with a lid and cook for 25–30 minutes, or until all the vegetables and beans have softened and cooked through.
Allow to cool slightly before whizzing with a stick blender in the pan. (If blending in a food processor, let the soup cool slightly, then ladle in small amounts and blend a bit at a time.)
Toast the pumpkin seeds in a small, dry, heavy-bottomed frying pan, heating them until they go crisp – about 3 minutes. Don’t let them burn. Take off the heat. Sprinkle these pumpkin seeds on top of the soup (about 6 each portion) and drizzle with a little oil and thinly sliced red chilli, if desired.