Divide the marinara, spinach, basil and cheese between each one.
Put them in a preheated oven at 200C/390F/Gas 6 for 20 minutes.
When cooked, gently drain any liquid.
To cook in the Instant Pot:
Assemble the pizzas as in steps 1-2 above.
Add a cup of water to the Instant pot and put the mushrooms on a trivet inside the pot.
Close the Instant Pot lid and turn the valve to Sealing. Press the "Pressure Cook" button and set the time to 10 minutes on High pressure.
When the Instant Pot is done cooking, natural release the steam for 5 minutes and then quick release.
Remove the Pizzas carefully from the Instant Pot and serve.
Mushrooms have a high water content so these do create a pool of liquid when cooking. Just gently pick them up and drain the liquid off before serving and you are good to go. You could bake them before filling them and drain the liquid before filling.
Prick holes in the bottom of the mushroom cap with a toothpick to allow the moisture to drip out, but not the toppings.
Chop any vegetables for the topping as small as possible. They are only in the oven for a little while, so you want to make sure they are cooked through.
Sprinkle with some fresh basil before serving.
You can make mini versions of these by using mini portobello mushrooms. These would be great for party food.