Grease a 2- to 3-quart microwave-safe dish lightly, then add the chopped apples.
Toss apples with brown sugar, cinnamon, and salt until evenly coated.
Stir in melted butter, vanilla, and water to moisten the mixture.
Cover tightly with a microwave-safe lid or plate.
Microwave on High for 5–7 minutes, stirring halfway, until apples are tender but not mushy.
Let stand covered for 2 minutes. Stir in oats, nuts, or raisins if using; microwave 30–60 seconds more to warm them through.
Serve warm. Spoon over yogurt, oatmeal, or ice cream if desired.