Drain and wash the seitan. (I then boil water in a pot and place the seitan in it and let it simmer for about 3 to 5 minutes, just to soften it up. I don't know if this is necessary though. I then drain the seitan and let it cool.) Cut the seitan into bite size pieces.
In a large container mix the vegan sour cream and lemon juice.
Add the garlic, ginger, and onion to the sour cream/lemon juice, and mix well.
Add all of the dry seasonings to the sour cream mixture: paprika, salt, cumin, turmeric, coriander, garam masala, and cayenne. Mix well.
Add the seitan to the sour cream mixture and mix well so the mixture covers all of the seitan.
Cover the container and let it marinate in the refrigerator overnight or for at least an hour.
Place a little coconut oil in a large pan (I'm sure olive oil or another oil would be fine to use instead). Place all of the seitan pieces in the pan and cook on medium-high until the sides of the seitan start to get a little brown. I cook it for about 15-20 minutes, but it's possible to cook it until it is even more golden (more tandoori-like)--matter of taste. Turn the seitan over occasionally.
Pour the rest of marinade on top of seitan and cook until everything is heated up.
Serve on brown rice (or rice of your choice).