What religion doesn't eat mustard?

Well there are some religions that don't eat meat, there's Sikhism, Hinduism, Buddhism, and some derived modern movements. Such oriental religionists do not eat meat because of their belief in the sacredness of ANY life, so that they do not kill even rats or ants.

There are also Christian churches or groups that strongly encourage vegetarianism, because of the importance between physical and spiritual health (Seventh-day Adventists and related groups), or as ascetic exercise (fasting), as some Catholic or Orthodox monastic order etc.