1: Prepare 1 kg mussels as for mussels marinière, then shell them. Keep 1 cup of the cooking liquid. Note: for a simplified version, cook on high heat in a large stewpan with a small amount of water in the bottom, just until the shells open. Remove the shells.
4: Heat 5 tablespoons olive oil in a large frying pan, then add the onion and garlic, salt and pepper. Cook for 1 minute without colouring.
5: Add 250 g rice and stir frequently for 1 or 2 minutes, until you see the rice become translucent (or "pearled").
6: Pour in 400 ml chicken stock all at once, mix well and leave to cook on low heat until the rice has absorbed all the stock (about 30 minutes).
7: After this time, the rice should be cooked, but not dry. Add a little more stock if necessary.
8: Add 8 g squid ink and the liquid from cooking the mussels, if you have kept this. Stir to mix and colour the rice. Cover and leave to cook gently for 5 minutes.
9: Reheat the mussels rapidly. Heat the plates. Serve the mussels on a bed of arroz negro.
Remarks
I recommend using a round Spanish rice if possible, like used for paella.