Melt some butter or oil in a large soup pot over medium-high heat. Remove the casings sausages or brat links and brown on all sides for a few minutes. Try to keep them whole if possible while browning, but if you're using bulk just break into chunks as you cook. They do not need to be cooked all the way through, just till they start to stiffen up and get a golden color on the outside. Remove the sausage to a plate and cover with foil.
*A tip for washing and chopping a leek: cut off the dark green parts, wash well, and reserve for making stock later. Stand the rest of the leek upright on a cutting board, holding the root end, insert the knife into the leek (keeping the root end intact) and slice vertically downwards. Turn the leek a quarter turn, and make another perpendicular slice down. Fan out the quartered leek in running water to wash away the grit, then place on cutting board and chop finely.
In the juices that are left over in the pot, add your minced leek. Cook for 5 minutes over medium-low heat, till yellowing and slightly softened.
Add the salt, garlic powder, oregano, white pepper, kohlrabi and potato chunks and stir well. Cover and steam for 5 minutes, then add the wine and scrape up anything off the bottom. Recover and cook for 10 more minutes.
Add the stock and the bay leaf, bring to a boil, then reduce to a simmer. Allow to cook until the potatoes and the kohlrabi are totally soft (about 30 minutes.)
Remove and discard the bay leaf. Purée the soup in a standup blender like a Vitamix or using an immersion blender till totally smooth. Pour back into the soup pot if you puréed in a blender and set over medium-low heat. Taste and adjust seasoning by adding an extra splash of wine, salt or pepper, and add more stock or water to thin if needed.
Slice your sausages into small coins if they're still in links and add to the soup along with the kale. Stir till softened and sausages cooked through. Serve warm.