Over medium heat, add 1 T oil + 1 T butter to 8 quart pot. Saute' onions until soft and transparent add garlic and saute' for additional minute. Add leeks, potatoes, kohlrabi if using, and stock. Bring to a boil and immediately reduce heat to simmer. Partially cover pot and simmer until vegetables are completely soft, about 45 minutes.
Using a hand blender or blender, puree soup until desired consistency. Completely smooth for a classic version or leave it a little chunky for a more rustic soup.
Add cream heat to warm through (for a lighter version omit cream.)
If soup is to thick, add broth to desired consistency
Salt and pepper to taste.
Garnish with bacon, pancetta or chives and a dollop of creme fraiche' or crab meat if a seafood stock is used.