This is a dish that's developed over the years, by necessity. I started making it as a traditional caldo verde, using leafy kale and Portuguese chouriço. One day, when I was craving the soup, I couldn't find leafy kale in any of the markets, so substituted Chinese kale (kai lan). It was delicious. The next time, I didn't have any chouriço or Spanish chorizo, so I used another firm, fatty sausage: Chinese lap cheong. Again, it was delicious.
I don't think a thrifty Portuguese grandmother would protest too much at the substitutions. After all, caldo verde isn’t an haute-cuisine soup; it’s just simple, hearty and delicious.
Cut the onion in half, then chop it into medium dice. Halve the garlic cloves then thinly slice them. Peel the potatoes and cut them into medium-size cubes. Slice the thicker kale stalks into 1cm pieces, and leave the thinner stalks and the leaves whole. Roughly chop the coriander leaves.
Slice the Chinese sausages on the diagonal into pieces about 5mm thick. Put the pieces in one layer in a medium-size soup pan and place it over a medium flame. Cook the sausage pieces on one side until they start to brown, then turn them over and brown the other side. Use chopsticks to remove the sausage pieces from the pan, leaving behind all of the fat.
Add the onion and garlic to the soup pan, sprinkle lightly with salt and cook over a medium-low flame until the onion starts to soften.
Add the potatoes and stir to coat lightly with the fat - if necessary, add a little cooking oil.
Pour in about 300ml of chicken stock and bring to a boil over a medium-high flame. Lower the heat and simmer until the potatoes are almost tender. Stir in more chicken stock to achieve the correct consistency.
Add the pieces of thick kale stalks and simmer for about two minutes, then stir in the thinner stalks and the leaves. Simmer for a few minutes, or until the kale stalks are tender.
Stir the Chinese sausage into the soup and simmer for about two more minutes.
Season with plenty of freshly ground black pepper, then taste the soup and add salt, if necessary.
Ladle the soup into bowls and scatter with the chopped coriander. Serve immediately.