NestJS has added a waitForCompletion option in version 5.0.1.

https://github.com/nestjs/schedule/pull/1870

@Cron(CronExpression.EVERY_SECOND, {
  waitForCompletion: true,
})
async handleCron() {
  this.logger.debug(`start ${c}`);
  await timeout(3000)
  this.logger.debug(`end ${c}`);
  c += 1;
}