Some variations include wild onion instead of cumin, while others also include radhuni seed in addition. However, panch phoron is a slight misnomer when applied to blends that include radhuni (Trachyspermum roxburghianum syn. Carum roxburghianum), since in Oriya or Bengali panch phoran literally means "five spices".
In the tradition of Oriya and Bengali cuisine, one usually first fries the panch phoron in cooking oil or ghee, which causes them to start popping immediately. At this point, one adds vegetables (especially potatoes), lentils, or fish to the cooking vessel to coat with the spice mixture.
In Bengal, the cradle of this mixture, a spice called radhuni is used, not mustard seed. A better replacement for radhuni, hardly available outside of Bengal, would be celery seed.
Panch phoron is usually not used to season any meats other than fish.