Saute onion in half the butter in large frypan until golden.
Place in the crockpot/slow cooker.
Mix together the beef, eggs, salt and pepper.
Roll into small balls.
Brown meatballs in same frying pan.
Sprinkle with tarragon, marjoram and flour.
Shake the pan to turn meat balls, this coats them with
the flour mixture.
Place into crockpot/slow cooker.
Mix together tomato paste, beef stock, worcestershire sauce
and vinegar in the pan. Scrape the bottom of the pan to
get any loose pieces up and cook for 2 minutes.
Pour over meatballs.
Cover and cook on low for about 1 1/2 hours.
Melt remaining butter in same pan and saute mushrooms
for a few minutes.
Add mushrooms and the sour cream to crockpot/slow cooker
and let heat through.