use Symfony\Bridge\Twig\Mime\NotificationEmail;

$email = (new NotificationEmail())
    ->from('fabien@example.com')
    ->to('fabien@example.org')
    ->subject('My first notification email via Symfony')
    ->markdown(<<<EOF
        There is a **problem** on your website, you should investigate it
        right now. Or just wait, the problem might solves itself automatically,
        we never know.
        EOF
    )
    ->action('More info?', 'https://example.com/')
    ->importance(NotificationEmail::IMPORTANCE_HIGH)
;