I am not sure why this rather intriguing paste like looking thing is called this way. But it is essentially raw minced beef. As Expat Support educated me (I had to ask!), most Dutch people eat it spread on toast or in a sandwich with raw onions. Although you can fry it too. Tartar is also quite popular here, the other day I almost bought a sandwich with it in De Broodzaak (similar to Pret in the UK). I always respect unique cultural differences, but I am not sure I will be trying filet americain any time soon…