Add sausage to a large stockpot and cook over medium-heat for 5-6 minutes, stirring and flipping occasionally, until the sausage is lightly browned. Use a slotted spoon to transfer the sausage to a separate plate, and set aside.
Add the olive oil, leeks, carrots and celery to the pan, and stir to combine. Sauté for 5 minutes, stirring occasionally. Then add the cabbage and garlic, and sauté for 4 more minutes, stirring occasionally.
Add the stock, potatoes, Italian seasoning, bay leaf, cooked sausage, and stir to combine. Continue cooking until the soup reaches a simmer. Then reduce heat to medium-low, cover, and simmer for 15 minutes, or until the potatoes are cooked and tender. Taste and season with a few generous pinches of salt and black pepper as needed.
Serve warm. Or refrigerate in sealed containers for up to 3 days, or freeze for up to 3 months.