you can do either ;-)
i'd probably choose to freeze them uncooked. cook the ingredients that you need to make the rissoles, but before you do the last step in cooking them, wrap them up and then to cook you can take them out to defrost for a bit and then bake but extend the cooking time. if you are going to pan fry them to make, then you can defrost for a little longer so they're not frozen solid and then just do on the pan a bit more till they're done.
otherwise you can cook first and freeze the leftovers, but they will probably be fresher the other way.