Preheat the oven to 350°F. Quarter the apples, remove cores, then chop into 1-inch pieces or thin slices, leaving the peels on if desired. Arrange the prepared apples in an 8×8-inch or 2-quart baking dish. Sprinkle with brown sugar, cornstarch, lemon juice, vanilla extract, cinnamon, and nutmeg, then gently toss to coat the fruit evenly. In a large microwave-safe bowl, melt the butter, then whisk in brown sugar, cinnamon, and salt. Stir in flour, oats, and optional nuts until a crumbly mixture forms. Evenly distribute the oat topping over the apple mixture. Bake for 40–45 minutes until the topping is golden and the filling may bubble at the edges. Allow to rest for 5–10 minutes before serving warm with a scoop of vanilla ice cream, a dollop of whipped cream, or a spoonful of yogurt.