Cook the broccoli and cauliflower in the broth and water for 1-2 hours at low-medium heat, until soft. Purée with a hand blender in the pot, or transfer to a food processor to purée then return to the pot.
In a small pan, sauté the onion and oil until translucent and lightly browned.
Add to the broccoli-cauliflower mixture. Season with cumin, salt and pepper, cooking further only until it has cooked down to the consistency you like.
Serve garnished with parsley, cilantro or mint.