Mussels steamed in their own juices are the Belgian classic at its plainest, cooked in one pot in about ten minutes. Most of the work is the cleaning; the rest needs only a heavy 5-litre pot with a tight lid. For two, you need 2 kg of mussels, two onions, two celery stalks, two small sprigs of thyme, two bay leaves, a small bunch of flat-leaf parsley, a pinch of salt and black pepper.
scrub and debeard the mussels
Rinse the mussels under cold running water. Pull each beard firmly towards the hinge until it comes away and scrape off barnacles with a knife. Tap any open mussel on the counter; if it does not close within a minute, throw it away, along with any cracked shells.
slice the onions and celery
Peel the onions, halve them and cut them into thin half-moons. Slice the celery into 5 mm pieces, leaves included. Chop the parsley coarsely and set it aside. Keep everything raw; the vegetables cook in the steam from the mussels.
build the pot in layers
Tip half the mussels into the cold pot and cover them with half the vegetables. Add the thyme, bay leaves, salt and a good grind of black pepper. Top with the rest of the mussels, then the rest of the vegetables. Add no water; the mussels release their own liquid.
steam over high heat
Put the lid on and set the pot over high heat. After 4 minutes, hold the lid down with a tea towel and shake the pot hard. Steam for another 3 to 4 minutes, shaking once more, until nearly all the shells have opened wide. Do not wait for the last few.
finish with parsley and serve
Scatter the parsley over the mussels and give the pot a final shake to mix the layers. Discard any that stayed shut, along with the bay leaves. Bring the pot to the table with a bowl for empty shells and ladle the liquid over each serving. Belgian fries or crusty bread for the juices are all it needs.
The usual mistake is a pot that is too small: fill it no more than halfway, or the top layer stays shut.
