Heat oil in a medium saucepan over medium. Add cashews and cook, stirring often, until golden brown in most spots, 4–6 minutes. Add curry paste and cook, stirring often, until slightly darkened, about 3 minutes. Add tomato paste and cook, stirring, until mixture starts to stick to bottom pot, about 2 minutes. Add tomatoes and a splash of water and cook, scraping any browned bits that may have stuck to bottom of pot, until tomatoes are completely broken down and mixture is thickened, 7–10 minutes.