Rinse and soak basmati rice in water for 20 minutes. Strain before using.
Set Instant Pot to sauté on high. Add the carom seeds or cumin seeds, then add the onions. Stir in a pinch of salt.
Sauté onions until they are soft and caramelized. This will take a few minutes. Do get this right because the browned onions add lots of flavor. Add ginger garlic paste and mix it in.
Stir in the tomato paste and mix. At this point, if you have stuck bits of onions or tomato at the bottom of the pot, add ¼ cup of the mushroom stock or water and deglaze the pot. This will help avoid a "burn" message. Stir in the vegan yogurt followed by the mint and cilantro. Add the potatoes and carrots and sauté for a couple of minutes.
Add in the remaining vegetables - zucchini, bell peppers and mushrooms. Add biryani masala and cayenne and mix them in.
Add the strained rice to the pot, mix it with the veggies, then add the mushroom stock or water. Add enough salt so the stock tastes a little saltier than you'd like the dish to be.
Layer on the optional ingredients at this point -- the fried onions and the mint - if using.
Turn off the sauté function and place the lid in place on the Instant Pot. Pressure cook on high for seven minutes. Let the Instant Pot stand 10 minutes after cooking has ended, then do a force-release.
Use a fork to lightly mix the biryani and fluff up the grains of rice.