Prep lentils & rice: Sift through the lentils and rice to discard any debris. Rinse and drain.
Sauté: Place the stainless-steel inner pot in the Instant Pot, select Sauté, and press Start. Let the pot heat for 1 minute, then add a splash of the water, serrano pepper, and ginger. Cook until softened, about 2 minutes. Press Cancel.
Add: Add the rest of the water, lentils, rice, cauliflower, cumin, garam masala, coriander, turmeric, pepper, cayenne (optional), and a pinch of salt (optional). Stir well.
Pressure cook: Lock the lid, select Pressure Cook on High for 23 minutes, and press Start.
Pressure release: When the timer beeps at the end of 23 minutes, let the pressure release naturally for at least 10 minutes. Then, quickly release the remaining pressure by setting the Pressure Release knob to Venting (a wooden spoon helps keep your hand free from hot steam). After the float valve drops, remove the lid.
Finish & serve: Add the lemon juice and stir in the spinach until wilted. The kitchari will have a soft, fluffy porridge-like consistency. Adjust seasoning with more lemon juice, spices, pepper, and/or salt as needed. Serve warm.