web.php

<?php

/*
|--------------------------------------------------------------------------
| Web Routes
|--------------------------------------------------------------------------
|
| Here is where you can register web routes for your application. These
| routes are loaded by the RouteServiceProvider within a group which
| contains the "web" middleware group. Now create something great!
|
*/

$router->get('/notification', function () {
    $notification = (new \App\Notifications\InvoicePaid)->toMail('test@example.com');

    $markdown = new \Illuminate\Mail\Markdown(view(), config('mail.markdown'));

    return $markdown->render($notification->markdown, $notification->data());
});