The even money favourite, ridden by Andrew Lynch, was the class of a field of just three after Bog Warrior's withdrawal.
Fellow rising star Sir Des Champs was second with the distant Rubi Light bringing up the rear.
Flemenstar jumped immaculately over the two-and-a-half mile trip and stretched clear on the run to the second last and finished a comfortable five lengths clear.
Trainer Peter Casey said: "I'm delighted with that, and he's still not ready!
"He'll go to Leopardstown at Christmas and will get three miles. I've never had any doubt about him getting three miles.
"We'll have to see how he comes out of this race first. He'll be entered for the Paddy Power Dial-A-Bet Chase as well as the Lexus.
"I don't want to get to the bottom of him at this stage and, if we feel it's best for the horse, he could go for the two-mile race at Christmas.
"The (Cheltenham) Gold Cup is still the plan long term."