Preheat an oven to 200C/390F/Gas 6.
For the chorizo, pepper and couscous stuffed marrrow, slice the marrow lengthways and scoop out all the seeds, then place on a roasting tray.
Place the couscous in a bowl along with the lemon juice and enough boiling water to cover.
Cover with cling film and allow to soak for five minutes.
Meanwhile, heat a frying pan until hot then add the olive oil. Add the red onion and stir-fry for 3-4 minutes until just softened. Add the chorizo and fry for a further two minutes until just crisped and the juices are released. Add to the couscous, along with the pepper, tomatoes and herbs.
Mix well and season to taste with salt and black pepper. Spoon into the centre of the two marrow halves.
Place in the oven for 20 minutes and cook until piping hot and the marrow is just tender.
To check if the marrow is done, place the tip of a knife into the side, if it offers just a little resistance it is done.