Bitterballen should always be fried from frozen and here are some options on how to fry them:
My preferred method to fry Bitterballen is in a deep fryer at 160 C for 6 minutes, if you don't have a deep fryer, you can also use a pan with enough vegetable oil to cover the bitterballen, you may need to turn the bitterballen manually to get an even fry.
Alternatively various clients have advised me that an Air Fryer also works. Brush the bitterballen with a spoonful of vegetable oil and airfry on high heat until golden brown. (Thank you Ren for the Air Fryer feedback)
Bitterballen are served in the Netherlands with mustard to dip, and I highly recommend it as a excellent combination