Start by cleaning the green cabbage. Cut off the base using a knife and remove the outer leaves.
Remove the core and any damaged leaves. Cut the cabbage in half, then into quarters, rinse thoroughly, and discard any remaining tough outer leaves if needed.
Cut the cabbage leaves into thin strips and set aside.
In a large pot, melt the butter (you may add 1 tablespoon of olive oil if desired).
Add the chopped onion and sliced leek, and sauté for a few minutes until soft and fragrant, without browning.
Add the diced potato and sliced cabbage to the pot.
Pour in the water and add the vegetable stock cube.
Bring to a boil, then reduce the heat, cover, and let simmer for about 35 minutes, or until all the vegetables are tender.
At the end of cooking, you can either keep the vegetables chunky for a rustic soup or blend everything with an immersion blender for a smooth texture.
Adjust the seasoning with salt and black pepper to taste.
Serve hot, with a drizzle of heavy cream if desired for extra creaminess.