If you are already a knife-expert, you can skip this section, but if you are a newer cook: the easiest way to mince shallots that I’ve found that works for cooks of just about any skill level is to use a very small and thin knife, cut it once or twice horizontally, then every 1/8″ vertically, then slice every 1/8″ or so.
This is a home cooking kinda thing though, so without a head chef around to yell at you I don’t think it’s very important to be ultra exact. I certainly feel free to slice the shallots however which way I like, and you should too.
But it also means that you have to be diligent about prep, because the dish goes so fast that if you stop to cut something or look for Worcestershire in the depths of your pantry, you might find your sauce evaporated into basically nothingness (true story). It’s best to have all of your aromatics pre-cut in advance and all your sauces and mustards at hand. You don’t need to have everything pre-measured like a 1990s TV chef, but you don’t want to be running around like a headless chicken, especially if this is for a date or dinner party, especially if you are trying to exude confidence in the kitchen.