This traditional Portuguese soup is thick with potatoes and kale. In this version, I substitute kelp for the kale to add a nuanced ocean merroir. Find this and more in The Joy of Seafood.
Heat two tablespoons of olive oil over medium heat in a large saucepan. Add the onion and garlic and cook until soft, about 5 minutes.
Add the potatoes and sausage and cook until the potatoes begin to soften, about 5 minutes.
Add 6 cups of water and the kelp to the pan and bring the mixture to a simmer. Let it simmer for 15 minutes, or until the potatoes begin to fall apart.
Mash the potatoes with the back of a spoon and simmer for another 10 minutes.
Turn off the heat and add the mussels and season with salt.
Let it rest 20 minutes before serving.
Finish with a drizzle of olive oil and black pepper.