Barbecue Seitan Sandwich with Green Slaw

I know I've featured barbecue seitan here before, but it's one of my favorites. And last time, it lacked this most delicious accompaniment. This crunchy, tangy slaw is so lovely with the sweet and smoky seitan.

For the Slaw:
1/2 a green cabbage, sliced into thin strips
1 turnip, peeled and sliced into matchsticks
2 leeks, white and light green parts sliced thinly
1 green apple, peeled and sliced into matchsticks
1/4 cup parsley, chopped
1/4 cup veganaise (or mayonnaise)
1 tb apple sauce
1 tb dijon
2 tb apple cider vinegar

For the Seitan:
1/2 onion, chopped
4 cloves garlic, chopped
2 tb olive oil
1 8 oz package of seitan
1/2 cup barbecue sauce

Slaw:
Combine vegetables in a large bowl. Whisk sauce together and pour over the vegetables. Stir to combine thoroughly.

Seitan:
Saute the onions and garlic in the olive oil until the onions have softened. Add the seitan and continue to cook, stirring, until it's warmed through and slightly browned. Pour in the barbecue sauce and cook over low heat for about 5 minutes.