Cut the steak with a sharp knife as small as you can. You can do it with a food processor but it's really best to cut by hand for the best texture. It's easy to over process in the machine.
Mix the steak with the mustard, worcestersauce, tabasco, cornichons, shallot, fresh herbs and add pepper and salt to taste.
Prepare the serving by placing half of the steak tartare in a ring. Press down firmly and remove the ring. Serve the steak tartare with an egg yolk on top.
To do this make a dent in the top of the steak tartare and add the yolk there. You can also use the shell to place it on top. Make sure you clean the shell thoroughly before placing it on the meat.