Halve leeks lengthwise; rinse thoroughly,
pat dry, and cut into 1/2-inch
slices. In a large Dutch oven or heavy
pot, heat oil over medium-high. Add
onion, carrots, and celery; cook, stirring
occasionally, until softened, about
5 minutes. Add leeks and garlic and
cook, stirring, until leeks begin to
soften, 2 minutes. Add potato, broth,
and bay leaves and bring to a boil.
Stir in cabbage and season generously
with salt and pepper.
Return soup to a boil, then reduce
to a rapid simmer. Cover and cook until
vegetables are tender, 15 to 20 minutes.
Remove from heat; stir in vinegar
and thyme. Season with salt and pepper.