What is the best way to update one field with rows resulting 15k?

You can use chunk() for that. Quick example from the docs (http://laravel.com/docs/5.1/eloquent#retrieving-multiple-models):

User::chunk(200, function($users)
{
    foreach ($users as $user)
    {
        // Execute the row update here
    }
});

Or if all 15k rows should update with the same value:

Model::whereIn('id', $arrayOfIds)->update(['field' => $value]);

Yes, chunk and a queued command or event. Chunking is very efficient as the time to execute isn't significantly longer and doesn't use up your servers memory.