Here's my full recipe--this is just what I like but adding an egg yolk wouldn't be a terrible thing. I wouldn't add lemon or another acid though because you already have the caper/pickle which should add all the acid you need and I don't want the meat to turn brown and/or "cook".
Makes a nice "mound" that is good for 3-4 people--if you're doing as a more substantive "course" or even an entrée, adjust accordingly:
1/2 lb of highest quality tenderloin or filet--tell the butcher you're doing a tartare and they should be able to steer you to the best option
1 tbsp. finely chopped shallot
1-2 tsp finely chopped capers and/or cornichons/dill pickles (depends how much you like capers and/or pickled things--I use more because I like them)
1 tsp finely chopped Italian parsley
1-3 minced anchovies (see caper

)
1 tsp Dijon or coarse grain mustard
2-3 dashes of Lea & Perrins
1-2 dashes of hot sauce (anything but Tabasco--I think Tabasco tastes weird in it)--this is also optional--I often leave it out.
Coarse salt and ground pepper to taste
Hand chop the steak--I like to try to chop to about the same size as my other solid ingredients but you can also have the steak be a bit larger than the onions/capers. Depends how anal you are about it! Once that's done, using a non-reactive bowl, add in the solid ingredients, including the anchovy. Once those are incorporated, add the remaining ingredients and mix well. Serve with crostini or table crackers. I sometimes like it slathered on pieces of celery from my garden as well.
Enjoy!!!
"Knowledge is knowing a tomato is a fruit; wisdom is not putting it in a fruit salad." Miles Kington