Prepare the leeks: Cut the root end from the leek and discard. Cut off the dark green tops and reserve to use for another purpose. With the remaining white and light green piece, cut down the leek lengthwise, into two parts. With each part, slice into thin slices. Place into a fine-mesh strainer and using your fingers, separate the leek slices into individual pieces, then rinse well with cold water. Set aside to drain until needed. You will need one or two leeks, depending on their size.
Prepare the cabbage: Cut cabbage into quarters and remove the core. With each quarter, slice into thin slices, and then in half again if necessary, into thin slices no longer than 2 inches long. Remove any tough pieces. You will only need about 1/2 a head of cabbage. Set aside until needed.
In a large pot, over medium heat, cook the bacon until crisp, about 5 minutes. Remove bacon to a paper-towel lined bowl, leaving the rendered fat in the pot and the heat on under the pot.
Add the onions, leeks and cabbage to the pot and cook, stirring regularly, over medium heat until the vegetables are tender, about 10 minutes. Add the garlic and cook, stirring, an additional 30 seconds or so. Stir in the flour and cook, stirring, for 1 minute.
Add the chicken broth and potatoes and bring mixture to a boil over medium-high heat. Reduce heat to medium-low and simmer until the potatoes are tender, about 15 minutes. If soup seems like it doesn't have enough liquid, add a bit more chicken broth.
When potatoes are tender, remove pot from heat and stir in heavy cream. Taste, season with salt and pepper, as needed. Spoon soup into bowls and garnish with bacon and green onion.
Be sure to taste test your cabbage to make sure it's tender before adding the flour. It may take longer than 10 minutes.I used mini red potatoes, cut into quarters or sixths, depending on their size. You could use a diced russet (baking) potato, if you don't have or want to get red potatoes. The splash of red in the soup is very pretty though, if those things matter to you :)Be sure to taste and season well with salt and pepper at the end of cooking and before serving. A well-seasoned soup is a delicious soup!!