In a large stock pot, add olive oil over medium heat. Add carrots, celery, and onions. Season with a generous pinch of salt and the fennel seed or powder. Sautee for approximately 5 minutes.
Carefully cut the head of the cabbage in half. Remove the heart and slice the cabbage into desired size pieces. Add to the stock pot and season with a pinch of salt.
Stir to combine the cabbage with the vegetables in the pot. Cover and let the vegetable continue to cook down, for approximately 25 minutes. Check the heat, you may need to reduce to low heat depending on your stove.
Once the vegetables and cabbage begin to soften, add the chicken stock and fennel.
Increase heat to bring the stock to a slow boil.
Once the stock begins to slow boil, working with one sausage link at a time, squeeze the link to have the meat push out of the casing. Drop desired-size sausage pieces into the stock. I try to form a small meatball using just one hand as I add the sausage to the stock.
Once all the sausage is in the pot, reduce heat to simmer and let cook for at least one hour. Taste and add more salt if desired.