Get the portobellos marinating. In a shallow glass dish, whisk together the balsamic vinegar, olive oil, miso, garlic, mustard, salt and pepper. Remove the stems from the portobellos and scrape out the gills with a spoon. Place the portobellos, caps facing down, into the marinade. Spoon some of the marinade into the “cup” of the caps and spread it around. Let the portobellos marinate at room temperature for 20 minutes, flipping them over at the halfway point.
While the portobellos are marinating, make the kale caesar. In an upright blender, combine the cashews, garlic, lemon juice, capers, mustard, coconut aminos, salt, pepper, and water. Bring the blender up to high speed and blend until you have a smooth and creamy dressing. If it seems too thick, add a bit more water and blend again until you have a pourable, but still thick, consistency.
Place the chopped kale in a large bowl. Pour half of the cashew caesar dressing over the kale, along with some salt and pepper. Massage the dressing into the kale until every piece is coated. Taste a piece to see if you’d like more dressing, salt etc. Set aside.
Preheat a grill to high. Grab a flipping spatula and your portobellos. Place the portobellos on the grill, top side facing down. Pour the remaining marinade into the caps. Close the lid and grill for 3-4 minutes, or until slightly collapsed and charred on the edges. Flip the portobellos over. Close the lid and cook for 3-4 more minutes. The portobellos should be soft and juicy-looking.
Once you can handle them, cut the portobellos into strips. Divide the miso grilled portobello strips and kale caesar among the 4 warmed split pita. Add sliced tomato if you like. Enjoy the kale caesar pitas immediately!