`./artisan server` for Lumen 5.3. Kernel.php is in app/Console/ and ServeCommand should be created in app/Console/Commands/

Kernel.php

<?php

namespace App\Console;

use Illuminate\Console\Scheduling\Schedule;
use Laravel\Lumen\Console\Kernel as ConsoleKernel;

class Kernel extends ConsoleKernel
{
    /**
     * The Artisan commands provided by your application.
     *
     * @var array
     */
    protected $commands = [
        // Our serve command
        Commands\ServeCommand::class
    ];

    /**
     * Define the application's command schedule.
     *
     * @param  \Illuminate\Console\Scheduling\Schedule  $schedule
     * @return void
     */
    protected function schedule(Schedule $schedule)
    {
        //
    }
}

ServeCommand.php

<?php

namespace App\Console\Commands;

use Illuminate\Console\Command;
use Symfony\Component\Console\Input\InputOption;

class ServeCommand extends Command {

    /**
     * The console command name.
     *
     * @var string
     */
    protected $name = 'serve';

    /**
     * The console command description.
     *
     * @var string
     */
    protected $description = "Serve the application on the PHP development server";

    /**
     * Execute the console command.
     *
     * @return void
     */
    public function fire()
    {
        chdir(base_path('public'));

        $host = $this->input->getOption('host');

        $port = $this->input->getOption('port');

        $base = $this->laravel->basePath();

        $this->info("Lumen development server started on http://{$host}:{$port}/");

        passthru('"'.PHP_BINARY.'"'." -S {$host}:{$port} \"{$base}/public/index.php\"");
    }

    /**
     * Get the console command options.
     *
     * @return array
     */
    protected function getOptions()
    {
        return array(
            array('host', null, InputOption::VALUE_OPTIONAL, 'The host address to serve the application on.', 'localhost'),

            array('port', null, InputOption::VALUE_OPTIONAL, 'The port to serve the application on.', 10420),
        );
    }

}