Begin by washing and chopping your zucchini, bell peppers, and mushrooms into bite-sized pieces. Mince 2 cloves of garlic finely. Set your Instant Pot to 'Sauté' mode at medium heat.

In the hot Instant Pot, add 2 tablespoons of olive oil and let it heat for about a minute. Stir in the minced garlic, sautéing for about 30 seconds until fragrant. Then, add the chopped mushrooms and bell peppers, cooking for an additional 3-4 minutes.

After the vegetables have cooked down, add the bite-sized zucchini and packed spinach. Pour in one can of diced tomatoes, along with their juices. Season generously with Italian herbs, salt, and pepper.

Close the Instant Pot lid tightly and ensure the pressure valve is sealed. Set the pot to 'Manual' and cook on high pressure for 8 minutes. Allow natural pressure release for about 10 minutes.

If opting for cheese, stir in your desired amount of grated cheese while the casserole is still warm, mixing until it melts.

Once the cheese is melted and thoroughly mixed, your casserole is ready to serve! Ladle portions onto plates, garnishing with fresh herbs if desired.