Prep the Oven and Baking Dish: Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius) and line a baking dish with parchment paper. I used a deeper baking dish but I think a rimmed baking tray should also work fine. With the baking dish, I cut diagonal slits at each corner so I could line the walls of the baking dish too. This helped contain any mess and made cleanup a breeze.
Make the Cheese: If you haven't already, blend the cheese ingredients in the blender until smooth. Taste. It will taste a little chalky from the tapioca starch, but that flavor will go away after baking. Now is a good time to adjust the saltiness, spices, lemon, etc.
Clean the Mushrooms and Remove the Stems: Clean the Portobello mushrooms with a damp cloth, making sure to gently clean the gills and inside rim as well. Use a small knife to very carefully remove the stem. I save the stems to eat at another time (I chop them up and throw them in a cooked vegetable side or something).
Season the Mushrooms: Place the mushroom caps upside down in the lined baking dish. Add about a teaspoon of coconut aminos (or balsamic vinegar or unsalted vegetable broth if totally salt-free) to each mushroom cap and use a silicone basting brush or your fingers to gently brush the whole mushroom with the coconut aminos. Sprinkle dried Italian seasoning on each mushroom cap.
Assemble the Portobello Pizzas: Add a spoonful of pizza sauce to each mushroom cap. Then place a large spoonful of the cheese over the top of where you just put the sauce. Finally, sprinkle your toppings onto each pizza.
Bake: Place the baking dish on the center rack in your oven and bake for 20-25 minutes until the cheese looks solidified, the veggie toppings have softened, and the mushrooms look cooked. Remove from the oven and allow to sit for 5 minutes.
Serve: Carefully scoop out each pizza and place on a plate. For a full meal, my partner and I each eat 3 pizzas along with a starch like a serving of air fried French fries (recipe coming soon).