Dice 1 large yellow onion. Thinly slice 8 large garlic cloves. Peel 1-inch fresh ginger and coarsely chop.
Heat a 6-quart or larger Instant Pot or electric pressure cooker on the Sauté setting. Add 4 tablespoons unsalted butter and cook until melted. Add the onion, garlic, ginger, 3 tablespoons sweet paprika, 1/2 to 1 teaspoon cayenne pepper, 1 teaspoon ground coriander, 1 teaspoon garam masala, and 1 teaspoon kasuri methi if using. Cook, stirring frequently, until the onions begin to soften and the mixture is fragrant. Pour in 1/4 cup water and stir vigorously, scraping up any browned bits on the bottom of the pot.
Add 1 (28-ounce) can whole peeled tomatoes and their juices, 1/4 cup roasted cashews, 1 tablespoon granulated sugar, and 1 teaspoon kosher salt. (No need to stir.) Nestle 1 1/2 pounds boneless, skinless chicken thighs into the sauce.
Lock the lid on and make sure the pressure valve is set to seal. Set to cook for 6 minutes on HIGH pressure. Meanwhile, juice 1 to 2 medium limes until you have 2 tablespoons juice, then cut the remaining lime into wedges for serving. Coarsely chop the leaves and tender stems of 1/2 bunch fresh cilantro until you have 1/4 cup.
When the cook time is up, quick release the pressure. Using tongs, transfer the chicken to a clean cutting board. Transfer the sauce to a blender and add 1/4 cup heavy cream. Blend until smooth, then return to the pressure cooker. (Alternatively, add the cream to the sauce and blend directly in the pressure cooker with an immersion blender.)
Cut the chicken into 1-inch pieces. Return to the sauce and stir to combine. Cook on the Sauté setting, stirring frequently, until simmering, about 5 minutes. Add the lime juice and stir to combine. Taste and season with more kosher salt as needed.
Transfer to a serving dish and garnish with the cilantro and a drizzle of heavy cream. Serve immediately with warm naan or flatbread and lime wedges.
