When making steak tartare, make sure to buy fresh meat, and chop or scrape it yourself. I prefer chopping the meat, as that gives the dish some more structure.
Finely chop the meat. Transfer to a medium bowl, and add celeriac, celery, sorrel oregano, mustard and salt. Grind the pepper and coriander with a pestle and mortar, and add to the mixture. Mix until smooth, and thoroughly blended.
Place the meat on a cutting board, and divide in four equal pieces. Shape nicely, transfer to serving plate, and top with egg yolk. Serve immediately.