There are no quantity restrictions regarding the use of broccoli in the Dukan Diet, thus you can eat as much broccoli as you want. In any form you like.
As to cooking methods, steaming without salt let broccoli retain most of its valuable vitamins and minerals.
You can also eat it raw if you like, just remember to rinse the vegetable thoroughly before doing that. BTW: Did you know that the edible part of broccoli is actually a flower?
Evidence say that certain compounds in brocolli have anti-cancer properties.
Amount Per Serving
Broccoli (Boiled without salt) Nutrition Data →
Only 1/7th of brocolli’s carbohydrate contents is sugar, and almost half of the carbs is dietary fiber.
Broccoli is the great source of Vitamin C (one 100g serving contains 100% of an average daily intake).