Start by making the cabbage: Set a deep, large pan over medium-high heat. Add the olive oil.
Add the cabbage, leeks, salt and sugar. Immediately turn the heat down to low. Stir every 8-10 minutes (be careful not to interrupt the browning process by moving the cabbage around a lot) until the mix turns a deep brown, about 40 minutes. If at any point the pan looks too dry, gradually add a little more olive oil.
Once cooked, set the cabbage aside in a bowl lined with a paper towel. Pat down with more paper towel to remove any excess oil.
To make the broth, heat a large pot over high heat. Brown the flanken in batches, returning the meat to the pot when the last batch is done.
Add the vegetables, herbs, salt and water. Bring to a boil, cover with a lid and turn down to a simmer. For the first 15 minutes, check on the broth to remove any scum that rises to the top. I like to skim off some of the fat as the flanken simmers, but you may find it easier to do this once the soup cools down and the fat solidifies.
After 1 1/2 hours, discard the vegetables and herbs, and add the beets, carrots and caramelized cabbage. Continue to cook for another 30 minutes, or until tender. Add salt and pepper to taste, and fresh herbs, if you’d like.