Preheat the oven to 350°F (175°C).
Place butter in a 9x13-inch baking dish and melt it in the oven while preheating.
In a saucepan, combine peaches, sugars, lemon juice, cinnamon, and nutmeg. Simmer for 5 minutes until syrupy. Add cornstarch if a thicker filling is desired.
In a bowl, whisk together flour, sugar, baking powder, and salt. Gradually add milk until a smooth batter forms.
Pour the batter evenly over the melted butter. Do not stir.
Gently spoon the warm peach mixture over the batter.
Bake for 40–45 minutes, or until golden brown and bubbling.
Serve warm with ice cream or whipped cream.