In a blender, puree the tomatoes with their juices until smooth, about 1 minute; transfer to a medium bowl and set aside. Add the cashews and ¾ cup water to the blender, then puree until smooth, about 1 minute; set aside if pressure cooking or cover and refrigerate if slow cooking. In a large bowl, whisk together the garam masala, cumin, ginger and 2 teaspoons salt. Add the chicken and toss to coat; set aside. On a 6-quart Instant Pot, select More/High Sauté. Add the butter and let melt. Add the onion and cook, stirring occasionally, until softened, 3 to 5 minutes. Add the garlic and cook, stirring, until fragrant, about 30 seconds. Stir in the chicken and tomatoes, then distribute in an even layer.