meteor-subscription-promise-ready.js

Template.restaurant.onCreated(function() {

    let restaurantSub = this.subscribe( 'Restaurant', [ FlowRouter.getParam('restaurantId') ] )
      , restaurantPromise = restaurantSub.readyPromise()
  
    this.restaurant = new ReactiveVar()
    this.ready = new ReactiveVar( false )
  
    restaurantPromise.then(function(){
      
      this.restaurant.set( Restaurant.findOne() )
      this.ready.set( true )
      
    }.bind(this))
  })

Thanks for the tip, much better!