Preaheat a nonstick pan over medium heat.
Place minced beef, eggs, chopped yellow onion, capers, caper juice, diced pickled beetroots, dijon mustard (if using) and salt to a bowl and mix until combined. But don't mix too long, just until combined.
Roll 25-30 equally sized meatballs. Add about 10-14 meatballs to the pan and fry for 3-4 minutes, until fried though. But don't fry for too long so they become dry. They should have a nice and golden brown surface, be cooked through but still not override or dry. I prefer to keep a liiiittle bit of a raw core, just because it makes such a lovely contrast to the fried surface. But you can also fry them so that they are fried all the way through, but they should still be pinkish in the middle. Otherwise they will feel bland and dry.