I attempted to use the Illuminate\Http\Request::fingerprint method, but it is another functionality documented in Laravel and inexplicably left out of Lumen by Mr. Otwell. Why am I not surprised that the documentation is so shoddy for yet another release of Lumen?

Full details of that error are in this issue: https://github.com/laravel/lumen-framework/issues/348

I created a replacement function that I appended underneath the helpers that Otwell arbitrarily and inexplicably left out of Lumen section of my bootstrap/app.php file.

function fingerprint()
{
    $r = request();
    return hash('sha1', json_encode([
        $r->url(),
        $r->method(),
        $r->ips(),
        $r->header(),
        $r->all()
    ]));
}

After some more fiddling/experimentation, I created this and put it in my bootstrap/app.php file.

// additional helpers
function cache ($key = '', $value = '', $minutes = 1)
{
    if (empty($key))
        return app('cache');

    if (empty($value))
         return cache()->has($key) ? unserialize(Serpent\Serpent::decrypt(cache()->get($key))) : '';

    cache()->put($key, Serpent\Serpent::encrypt(serialize($value)), $minutes);
}
function flash($key='', $value='')
{
    if (empty($key)) return '';

    $f = fingerprint();

    if (empty($value)) {
        if (cache()->has($f.':'.$key)) {
            return cache($f.':'.$key);
        } else
            return '';
    }

    if (!cache()->has($f))
        $c = [];
    else
        $c = explode(';', cache($f));
    if (!in_array($key, $c)) {
        $c[] = $key;
        cache($f, implode(';', $c));
    }
    cache($f.':'.$key, $value);
    return true;
}

Note that the Serpent library is a custom class for handling encryption.

All I have to do now is create a Middleware that cleans out the cache at the end of each request.