Heat olive oil in a large pot over medium heat.
Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and cook until fragrant, about 1 minute.
Combine the carrots and celery, cooking for another 3–4 minutes.
Add chopped cabbage to the pot and stir, cooking for about 5 minutes until slightly wilted.
Pour in the vegetable broth and add diced tomatoes, thyme, and bay leaf.
Bring the mixture to a boil, then reduce to a simmer.
Add salt and pepper to taste, and simmer for at least 30 minutes.
If using, add diced potatoes and cook for an additional 15 minutes until tender.
Remove the bay leaf before serving and adjust seasoning if necessary.
Ladle the soup into bowls and enjoy warm.