Add vinegar, soy sauce, mirin, ginger, oil, garlic, and tempeh to a frying pan; let mixture boil, stirring occasionally. Place lid on pan, lower heat, and simmer 15 minutes.
Take lid off of pan and raise heat to medium-high. Cook 4 minutes or until mixture is lightly browned. Let cool.
Heat a cooking spray coated pot over medium-high heat. Saute onion, ginger, salt, and garlic for 5 minutes. Mix in curry and turmeric and cook 1 minute. Let boil and cook 1 minute longer. Let cool for 5 minutes.
Using a food processor or blender, puree onion mixture with cashews and lemon juice until smooth. Serve tempeh drizzled with warm cashew sauce.