MeteorMethodInTemplateHelper.js

if(Meteor.isClient){
  
  // set Session variable in method callback
  Meteor.call('myMeteorMethod', "foo", function(error, result){
    Session.set('myMethodResult', result);
  });
  
  // use reactive Session variable in helper
  Template.myTemplate.helpers({
    myHelper: function(){
      return Session.get('myMethodResult'); // "bar"
    }
  });
  
}

if(Meteor.isServer){
  
  Meteor.method({
    myMeteorMethod: function(argument){
      return "bar";
    }
  });
  
}