Fricadellen, which are pan-sauteed patties of meatloaf-type mixture, were a staple of my German-born mother's cooking repertoire. She added chopped cooked cabbage to the seasoned beef/onion/bread/milk mixture. The cabbage "melts" into the meat, adding tender sweetness. It is not recognizable by either taste or appearance - you'd think it was onion. When I mistakenly thought I still had a cabbage wedge in the crisper, I made do with leftover coleslaw, which I rinsed, nuked, then cooled it before adding to the meat mixture. This was easier and tastier than plain cabbage. Next time, I skipped the rinsing, instead decreasing the milk a bit. Even better. Now, whenever I have coleslaw, I freeze some, thawing it the next time I make fricadellen, meatloaf, or meatballs. Freezing or cooking work equally well to break the cabbage's cell walls, which make it meld into the meat more thoroughly. I use about 3/4 cup of nuked or defrosted slaw per pound of meat, or about the same volume as the chopped onion in the recipe. If sauteeing for fricadellen or meatballs, be aware that the cabbage will scorch easily, so use medium heat and saute slowly, letting the patties/balls brown on the bottom before turning them. 4 to 5 oz patties will take 15-20 minutes total.
I know that the idea of cabbage, not to mention coleslaw, in meatloaf sounds very unappealing. Take a leap of faith (you could even reserve a little of your usual mix, adding just enough slaw for one or two meatballs as a taste test). Not only is it yummy, it adds fiber and nutritional value to the diets of none-the-wiser picky eaters.