For the filling

4 pounds peaches fresh (about 12 medium peaches), peeled and pitted, or about 9 cups frozen peach slices
3/4 cup granulated sugar
2 1/2 tablespoons lemon juice
2 tablespoons cornstarch
1/4 teaspoon ground cinnamon
1/4 teaspoon ground nutmeg

For the topping

1/2 cup unsalted butter melted
1 cup all-purpose flour
1/2 cup granulated sugar
2 teaspoons baking powder
1/4 teaspoon kosher salt
1 cup milk