Linsensuppe - German Lentils Soup

Yep, it is winter and I feel like cooking again :-) The other day I was craving for a hearty winter soup and I remembered this German dish that I haven't had for ages - Linsensuppe. After a quick google search I found this really easy recipe
200 g lentils
300 g potatoes
100 g carrots
100 g celeriac or celery
100 g leech
500 ml vegetable stock
500 ml beef stock
1 tbs butter
3 tbs balsamico bianco
boil the lentils in the vegetable stock until soft (about 30 - 45min)
chop carrots, celery, leech and the potatoes into small cubes
melt butter in a big pot and add the vegetables and sweat for about 5min. then add the beef stock, boil for 5 min.
add the lentils with vegetable stock
add vinegar to taste
I also add Wiener sausages from the German butcher - yum!