Preheat the Instant Pot by selecting the Sauté setting. Heat the oil in the preheated pot for a few seconds, then add the onion and a pinch of salt and cook until the onion is translucent, about 2 minutes. Add the ginger and garlic and cook for 1 minute. Stir in the pureed tomatoes, paprika, turmeric, garam masala, and remaining 3/4 teaspoon of salt and cook until almost at a boil, for about 2 minutes. Select the Cancel setting. Give it a good stir to pick up any stuck bits.
Add the potatoes to the pot and stir to coat them in the sauce. Gently pour in the water.
Lock the lid into place, set the Pressure Release Knob to Sealing, select the Pressure Cook/Manual setting at high pressure, and set the cook time to 9 minutes for 1-inch (2.5 cm) or smaller potatoes and 10 minutes for larger potatoes.
Once the cooking cycle is done, let the pressure release naturally for 5 minutes, then carefully set the Pressure Release Knob from Sealing to Venting to release any remaining pressure. Open the lid.
Stir in the cashew cream, select the Sauté setting, and cook to bring to a boil, about 2 minutes. Select the Cancel setting. Fold in the fenugreek leaves, if using. Taste for salt, adjusting if needed.