osb/app/Mail/TestEmail.php

<?php

namespace App\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Contracts\Queue\ShouldQueue;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;

use App\Models\User;

class TestEmail extends Mailable
{
    use Queueable, SerializesModels;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct(User $o)
    {
        $this->user = $o;
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
	    return $this
		    ->markdown('email.system.test_email')
		    ->subject('Just a test...')
		    ->with([
			    'site'=>$this->user->site,
			    'user'=>$this->user,
		    ]);
    }
}