Welcome to the Cake Decorators Q&A

Which will work best to put under the fondet ganashe or jam

Hello Poppy2014

Use ganache under fondant. Let it set before icing the cake, just make sure it is a little sticky so the icing sticks down properly. Jam is not a good idea, it will be very slippery and not a good medium to lay fondant on.
Hope this helps.