Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine the sliced peaches with brown sugar, granulated sugar, lemon juice, vanilla extract, ground cinnamon, ground nutmeg, and cornstarch. Mix gently for about 5 minutes.

Pour the peach mixture into a 9x13-inch baking dish, spreading it evenly.

Drizzle the melted butter over the peach mixture.

In a separate bowl, whisk together the all-purpose flour, baking powder, and salt until well combined.

Gradually stir in the whole milk into the dry mixture until smooth.

Pour the batter over the peaches, spreading it evenly while allowing peach juices to show.

Sprinkle the remaining brown sugar evenly over the top of the batter.

Bake for 40 to 45 minutes or until golden brown and bubbly.

Remove from the oven and cool for at least 10 minutes before serving.