public function onAfterRender()
{
    $url = (isset($_SERVER['HTTPS']) ? "https" : "http").'://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'];

    // Don't cache the admin area, maybe this can be improved also.
    if (strpos($url, 'administrator') !== false) {
        return;
    }

    $urlParts = parse_url($url);
    $uri = $urlParts['path'] !== '/' ? $urlParts['path'] : '/index';
    $uri = str_replace(['.html', '.php'], '', $uri).'.html';
    $file = JPATH_BASE . "/cache/static$uri";

    if (!is_dir(dirname($file))) {
        mkdir(dirname($file), 0777, true);
    }
    file_put_contents($file, $this->app->getBody());
}

RewriteEngine On

# Rewrite to html cache if it exists and the request is off a static page (no url query params and only get requests)
RewriteCond %{REQUEST_METHOD} GET
RewriteCond %{QUERY_STRING} !.*=.*
RewriteCond %{DOCUMENT_ROOT}/cache/static%{REQUEST_URI}.html -f
RewriteRule ^(.*)$  /cache/static%{REQUEST_URI}.html [L]

# This is specific to cache the index when it' accesed vie /
RewriteCond %{REQUEST_METHOD} GET
RewriteCond %{QUERY_STRING} !.*=.*
RewriteCond %{DOCUMENT_ROOT}/cache/static/index.html -f
RewriteCond %{REQUEST_URI} ^/$
RewriteRule .* /cache/static/index.html [L]