Select "All Cars & Tracks" at the bottom of the vehicle selection page. You will probably have to wait 10-20 seconds for all the content to load.
(Before doing this, you might want to have a look at the main settings page - press the cog in the top right-hand corner - to choose if you want Full Course Yellows, etc.)
The race setup page has four columns: Vehicle Track Opponents Settings
In the left-hand column, select the car you wish to drive. Next, choose the track you want. Under the Opponents tab, press the "Default" button to race against the same cars or choose from the list of cars above it. You can run any combination of any cars at any track. In Settings, arrange the race conditions how you wish.