The night before, cut up the onion, garlic, celery, and seitan and store in a large airtight container in the refrigerator.
In the morning, oil the crock of your slow cooker.
Combine all of the ingredients except the flour in the slow cooker.
Cook on LOW for 6 to 8 hours.
About 30 minutes before serving, adjust the stew's consistency. Add more water if the mixture is too thick, or add the flour if the stew needs to thicken up a bit.
Taste and adjust the seasonings.
Combine all the biscuit ingredients in a bowl and work until a dough forms.
Turn the mixture out onto a floured cutting board, roll out about ½ inch thick, and cut dough into circles with the rim of a glass.
Place biscuit dough circles on top of the stew in the slow cooker.
Turn up the slow cooker to high. Prop open the lid with the handle of a wooden spoon, or place a clean dishtowel under the lid to prevent condensation from dripping onto your biscuits. Cook an additional 30 minutes.
Notes
Add 1–2 cups (235–470 ml) extra water if you will cook it longer than 8 hours or if your slow cooker runs a little hot.