Heat oil or butter in a large pot over medium heat. Add chopped onion, diced carrot, and potato. Sauté for about 5 minutes until vegetables soften.
Add shredded purple cabbage to the pot and stir with the other vegetables. Cook for an additional 2-3 minutes.
Add cumin powder, black pepper, and salt. Stir well and bring the mixture to a boil.
Reduce heat to low and let the soup simmer for 20-25 minutes, or until vegetables are tender.
For a smoother texture, blend the soup using an immersion blender or regular blender. For a chunkier version, skip this step.
Stir in a teaspoon of lemon juice (optional). Garnish with coriander leaves and serve warm.