Melt 2 tablespoons butter in a large pan with a lid. Add 1 medium chopped onion and cook gently until soft, about 10 minutes.
Add 1 pound kohlrabi bulbs, peeled and chopped, and cook 2 minutes.
Add 2 ½ cups vegetable stock, 2 ½ cups milk, and 1 bay leaf to the pan and bring to a boil. Cover, reduce heat to low and simmer 25 minutes or until kohlrabi is tender. Let cool a few minutes and remove bay leaf.
Using an immersion blender , conventional blender or food processor, purée soup until smooth.
You may want to strain the soup through a fine sieve if the kohlrabi is especially fibrous.
Season to taste with salt and pepper. Serve in heated bowls with a hearty bread of choice.
