First create Rails app

rails new --database=postgresql --api nuxt-rails-a-la-webpacker 

Then the Nuxt app in Frontend folder

yarn create nuxt-app frontend  

Move package.json & nuxt.config.js to the root

mv frontend/package.json package.json  
mv frontend/nuxt.config.js nuxt.config.js  

And tell Nuxt the app is in frontend/ in nuxt.config.js :

srcDir: 'frontend/'

Set Nuxt to run on port 8080 : package.json

"config": {  
    "nuxt": {  
	    "port": "8080"  
    }  
},

Heroku config

We'll deploy an app for the backend and another for frontend to Heroku So we need 2 Procfiles and tell Heroku where the Procfile is located for each app

The Rails Procfile stay at the root :

touch Procfile 
echo "web: bundle exec puma -C config/puma.rb"  >> Procfile

The Nuxt Procfile is in frontend folder :

touch frontend/Procfile  
echo "web: npm start" >> frontend/Procfile  

Create frontend app on Heroku :

heroku apps:create nuxt-rails-client --region=eu  

and rename the remote frontend :

git remote rename heroku frontend  

Add multi-procfile buildpack

heroku buildpacks:add -r frontend https://github.com/heroku/heroku-buildpack-multi-procfile

Set the Procfile path

heroku config:set PROCFILE=frontend/Procfile -r frontend  

follow this guide to set Nuxt for production

Deploy the backend

heroku apps:create nuxt-rails-backend --region=eu  
git remote rename heroku backend  
heroku run rails db:seed -r backend

and add API_URL for @nuxtjs/axios heroku config:set API_URL=nuxt-rails-backend.herokuapp.com -r frontend