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; set aside.
Add olive oil, leeks, carrots and celery to the pan, stir to combine; sauté veggies for 5 minutes, stirring occasionally. Then add the cabbage and garlic, sauté for 4 more minutes, stirring occasionally.
Add the stock, potatoes, Italian seasoning, Herbs de Provence, red pepper flakes, bay leaf, and cooked sausage, 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.
Ladle soup into bowls. Serve warm with buttered bread for dunking.
Cook’s Note: Kielbasa smoked sausage is made with a variety of meats, beef, pork, chicken or turkey, your choice.