Prepare the Chestnuts: if using fresh raw chestnuts, score the flat side, roast at 200°C (400°F) for 20–25 minutes until the skins split, peel while warm and roughly chop; if using pre-cooked chestnuts, simply chop.
Heat the Olive Oil in a large soup pot over medium heat, then add the chopped Onion, sliced Leek and minced Garlic and sauté until soft and fragrant, about 6–8 minutes.
Add diced Carrot and Potato along with the chopped Portuguese Cabbage and the prepared chestnuts; stir to combine and cook for 2–3 minutes.
Pour in enough Water to just cover the vegetables, add the Bay Leaf, bring to a boil, then reduce heat and simmer gently until all vegetables and chestnuts are very tender, about 20–25 minutes.
Remove and discard the bay leaf. Use an immersion blender to purée about half the soup for a creamy texture while leaving some chunks for body (alternatively, transfer half to a blender, purée, and return to the pot).
Meanwhile, slice the Portuguese Chouriço and fry the slices in a small skillet until the edges are crisp and the fat renders, about 3–4 minutes; reserve a few crispy pieces for garnish and stir the rest into the soup to infuse flavor.
Season the soup with Salt and freshly ground Black Pepper to taste, and adjust consistency with a little more water if desired.
Ladle the soup into bowls, top with the reserved chouriço slices and a final drizzle of olive oil, and serve hot.
