Mix the breadcrumbs with the stock in a bigger bowl and let it soak while you continue with the onion
Finely chop the onion and fry it in some butter/oil until translucent
Add all ingredients in the bowl with the breadcrumbs and mix them together
Form meatballs from the meat mixture and put on a plate. Make them between 3 and 4 cm in diameter.
Fry in butter and allow them to get some color to really bring forward the meat flavours. Don't overcrowd your pan, make more batches instead. The meatballs are ready once they have colour all around.
You can make these in advance and right before serving you can heat them up in the oven
An alternative way that is a little easier is to cook the meatballs in the oven and then fry them after 10 min on 175°C (350°F) in the oven. Fry them on high heat for around 5 min to get some nice color and better taste.