Found it, I think this was suggested to me in another answer.
$output .= ...
ob_start();
settings_fields('my_settings');
do_settings_sections('my_settings');
$output .= ob_get_clean();
Found it, I think this was suggested to me in another answer.
$output .= ...
ob_start();
settings_fields('my_settings');
do_settings_sections('my_settings');
$output .= ob_get_clean();