Top and tail the marrow if necessary then cut into four lengthways and scoop out the seeds. Cut in to 1cm thick slices then lay on a tray and scatter with a little salt and leave for 20 minutes.
Meanwhile heat the butter in a pan and gently cook the onion and chorizo for 4-5 minutes, breaking it up with the back of the spoon as it’s cooking until it resembles mincemeat. Remove from the heat then stir in the breadcrumbs. If the chorizo hasn’t broken down you can give it a quick blend with the breadcrumbs in a food processor.
Pre heat a grill to maximum setting. Drain any liquid from the marrow slices and pat dry with kitchen paper then heat the oil in a large heavy reliable or non-stick frying pan. Season the marrow with pepper only then fry them on a fairly high heat for 4-5 minutes, turning them as they are cooking, until tender. You will probably need to do this in a couple batches, depending on the size of your pan.
Once cooked, toss the marrow in two thirds of the breadcrumb mixture and transfer to an oven proof dish and scatter the rest of the crumbs on top. Place the dish under the grill for a few minutes until nicely coloured. Meanwhile poach your eggs and serve on the marrow.