In a large, shallow bowl, whisk together the balsamic vinegar, oil, garlic, tamari, salt, and pepper. Place the portobellos in the balsamic marinade and flip them over to coat. Flip them so that the bottoms are facing up and spoon marinade into the caps. Set aside for 20 minutes, or up to 2 hours.
Make the ricotta. Drain and rinse the cashews. In a food processor combine the cashews, nutritional yeast, miso, lemon zest, lemon juice, maple syrup, garlic powder, onion powder, 5 tablespoons of water and salt to taste. Pulse the mixture until the cashews are finely chopped. Then, run the motor on high until you have a smooth, lightly textured ricotta-like mixture. Add more water by the teaspoon if necessary and scrape down the sides of the food processor bowl. Check ricotta for seasoning and set aside.
Preheat a grill to high. On a tray or large platter bring out your portobellos, the ricotta, any pizza toppings you’re using, and a flipping spatula. Place the portobellos on the grill top side facing down. Close the lid and grill for 3-4 minutes, or until slightly collapsed and charred on the edges. Flip the portobellos over.
Spoon tomato sauce, ricotta, and add any other toppings to the portobellos. Close the lid and cook for 3-4 minutes. Once filling is bubbling and portobellos are totally soft, juicy-looking, and evenly charred, carefully remove them from the grill. Serve pizza portobellos on their own or on fresh toasted ciabatta.