Mix tamari, cumin, chili powder, and water in a bowl large enough to hold the veggies. Set aside.
Wipe the portobello mushrooms and remove the gills.
Slice vegetables into long strips and put in the marinade. Marinade the vegetables for at least an hour.
Pour olive oil onto a cast iron skillet. Place the vegetables in the skillet. You can also use a grill pan or cook on a grill. Cook until tender, adding salt and pepper near the end of cooking. If using a grill or grill pan, cook until you see grill marks.
Serve with tortillas and salsa.
Video
Notes
I suggest assembling these mushroom fajitas right before serving. The marinated veggies will last up to 5 days in an airtight container in the fridge.