The fastest and easiest way to ruin any seafood dish is by overcooking. Unlike meat, seafood turns chewy and rubbery with overcooking. And they shrink like anything.
Most people make mussel soup by adding the mussels to the pan before pouring in the water. I don’t. I let the water boil first BEFORE adding the mussels. This way, I don’t overcook the mussels. After the water boils again, it only takes two to three minutes for the mussels to cook completely.
Wash the mussels under running water. Place in a glass bowl and cover with fresh water. Put in the fridge and soak to expel dirt, changing the water several times (see do mussels and clams need to be soaked before cooking?).
Discard the mussels that do not open. Wash under running water again. Pull out the “beards”. Set aside.
Heat a large pot. Pour in the cooking oil. Saute the onion until softened. Add the garlic and ginger and continue sautéing until lightly browned.
Pour in the water. Bring to a full boil. Do not lower the heat at this point. Add the cleaned mussels to the boiling water. When the water is boiling again, lower the heat. Season with patis and pepper. Stir. Cover and simmer for two to three minutes. Do not overcook the mussels. Just before turning off the heat, add the chopped greens. Serve at once.