Spread out the lentils and rice, searching for and discarding any rocks. Rinse and drain the lentils and rice in a colander.
Make sure the stainless-steel inner pot is in the Instant Pot. Select Sauté and press Start. Let the Instant Pot heat up for a minute, then add a splash of the water, the serrano pepper, and the ginger. Cook until softened, about 2 minutes. Push Cancel.
Add the rest of the water to the Instant Pot, along with the lentils, rice, cauliflower, cumin, garam masala, coriander, turmeric, white or black pepper, and cayenne pepper (if using). Stir well.
Put the lid on the Instant Pot and lock it in place by turning it to the right until you hear a beep.
Select Pressure Cook on High for 23 minutes and press Start.
When the timer beeps at the end of 23 minutes, let the pressure release naturally for at least 10 minutes. Then, use a wooden spoon to press the Pressure Release knob to quickly release any remaining pressure.
Once the pressure has fully released (you’ll know this has happened when the metal Float Valve drops), twist the lid to the left to unlock it.
Remove the lid. Add the lemon juice and the spinach. Stir until the spinach wilts. The kitchari will have a soft, fluffy porridge-like consistency. Taste and adjust the flavor to your preferences by adding additional spices, lemon juice, pepper, and/or a pinch of kosher or fine sea salt (optional). Serve warm.