Place the rack inside a 6-quart Instant Pot® and add 2 cups of water. Firmly pack the spinach inside the pot; it will be completely full and very tightly packed.
Follow the manufacturer's guide for locking the lid and preparing to cook. Set to pressure cook on high for 1 minute (see Cook's Note).
After the pressure-cook cycle is complete, follow the manufacturer's guide for quick release and wait until the quick-release cycle is complete. Being careful of any remaining steam, unlock and remove the lid.
Drain the spinach in a sieve and discard the cooking liquid. Transfer the spinach to a food processor and pulse until very finely chopped. Set aside and keep warm.
Wipe the pot clean and return to the base. Set to high saute, add the butter and heat, stirring, until completely melted. Add the paneer and cook, turning the cubes occasionally, until browned on a couple of sides, 6 to 8 minutes. Transfer to a paper towel-lined plate and set aside.
Add the ginger, garlic, onion, serrano, 2 1/2 teaspoons salt and 1/4 teaspoon pepper to the pot and cook, scraping up any browned bits, until the onions are tender, 6 to 8 minutes. Stir in the garam masala and cook until very fragrant, 1 to 2 minutes. Add the paneer and spinach with any accumulated juices and cook until heated through and the cheese is slightly softened, about 5 minutes. Turn off the heat and stir in the yogurt and lemon zest and juice; season with additional salt and pepper if needed. Serve over basmati rice or warmed naan.
Cultured butter has been treated with cultures and aged, resulting in a tangy flavor. European or regular butter can be substituted.
Settings may vary on your Instant Pot® depending on the model. Please refer to the manufacturer's guide.
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