This is unlocked upon earning the final Gold Trophy in the last available race completed in Career Mode.
For quicker results, have a fast tuned car and hire a driver to complete the race. If the Performance Index is higher then the rest of the opponents, then hire the LEAST skilled driver. Despite his unimpressive report card, he will win the race. This way you can still gain about half of the winnings and in less time.
Rinse and repeat if necessary, but this is mostly effective for the Endurance races.