Using butter or non-stick spray lightly coat a 7-cup heatproof glass container.
Create a sling from aluminum foil by folding a 20 inch long piece in half lengthwise twice. Center the glass container on the sling. Pour 2 cups of water into the Instant Pot and set the trivet inside.
In a large bowl, whisk together the eggs, milk, salt and pepper. Then stir in the spinach, mushrooms, garlic, tomatoes and cheese.
Pour the mixture into the glass container and cover tightly with foil. Using the foil sling that you created, lift the container and place it in the pressure cooker on top of the trivet. Fold over the edges of the sling.
Secure the lid and set the vent knob to sealing. Using the manual setting set the time to 25 minutes at high pressure.
When the machine beeps, let the pressure release naturally for 10 minutes and then do a quick pressure release by moving the vent knob to venting. Open the pot, and remove the container using the sling that you created and heat proof mitts. Carefully uncover the foil, it will be steamy.
Cut into wedges and sprinkle with parsley to serve.