PostController.php

<?php

class PostController extends Controller
{
    public function showAction($slug, $_template)
    {
        $post = $this->someBackend->findBySlug($slug);

        return $this->render($_template, ['post' => $post]);
    }
}

routing.yml

some_route:
  path: /blog/{slug}
  defaults:
    _controller: App:Post:show
    _template: post/single.html.twig