Peel and dice the onion. Cut off the root and the green part of the leeks and throw them away. Wash the white part thoroughly and cut into thin slices. Sauté the leeks and the onion in butter for 10 minutes or until opaque. Fry the bacon and set aside. Peel the potatoes and cut in 1/4 inch slices. Combine all ingredients except for the sour cream, Cheddar cheese and bacon in a large saucepan and simmer on medium heat until the potatoes are soft. Put the potatoes in blender or food processor (without the liquid) a small amount at a time and blend until smooth. Fold in the sour cream. Add the potato mixture back to the reserved soup and stir until combined. Add extra salt, pepper or sage to taste. Serve the soup garnished with Cheddar cheese and crumbled bacon. *Note: If you prefer to have potato chunks in the soup, set aside some of the potatoes before blending the soup. Add them back in once the remaining mixture is blended smooth.