Add the coconut or olive oil to the pressure cooker to cook over medium heat (press the 'Saute' button to begin cooking at medium saute heat). When the oil is hot, add the chopped onions and cook until they soften and become translucent. Stir in the garlic, and cook for another minute until fragrant. Turn off the heat (hit the 'Cancel' button), then add the cumin, coriander, turmeric and cayenne and mix well to combine.
Add the lentils, water, salt and tomato wedges and stir into the onion mixture. Cover the Instant Pot with the lid, and be sure that the valve is in sealed position. Press the 'Manual' button and adjust cooking time to 10 minutes. After the program is finished, press 'Cancel' to turn off warming mode, and wait 10 minutes before opening the valve to release pressure.
Remove and discard the tomato skins, and whisk together the lentils to emulsify, smashing the tomato wedges against the side of the pot if needed. Add the spinach, along with cilantro and/or butter if desired, and stir to combine. The residual heat will wilt the spinach quickly.
Serve over brown rice or with naan, topped with plain yogurt and fresh cilantro.