I find oven-baking the best method to cook forced rhubarb, as the pretty stalks remain intact that way. Forced rhubarb releases quite a bit of liquid - don't discard that, it is just as delicious as the rhubarb!
Cut the rhubarb into 5cm chunks and carefully mix it with the sugar in a large bowl. Place it in a large ovenproof glass or ceramic dish (no metal, as this will react with the acid in the rhubarb). Cover it with a large piece of parchment (no aluminium foil, as this will also react with the rhubarb) and secure it with kitchen string.
Cook for 25 minutes in the preheated oven, then remove the parchment and cook for another 10 minutes. Remove from the oven and let cool. Enjoy immediately or store in the fridge.