Brown the butter first as it will need to cool before using. Add 3/4 cup unsalted butter to a saucepan over medium heat. Stir with a silicone spatula constantly until the butter is melted. It will foam and pop slightly. Continue stirring until it smells nutty and turns amber in color, about 10 minutes. Pour into a bowl to cool.
Preheat oven to 350 degrees. Place apples into a large bowl. Add the lemon juice and vanilla and stir to coat. Then add the cinnamon, brown sugar, granulated sugar, flour and nutmeg. Stir well to evenly coat the apples. Pour into a glass or ceramic 13×9” baking dish.
In a medium bowl, add the flour, brown sugar, oats, baking soda, baking powder and salt. Whisk together well. Pour cooled brown butter over top of the dry ingredients.
Gently stir together until the topping is clumpy and mixed well. Using your hands, gently drop clumps of the topping over the apple filling. Bake for about 45-55 minutes until the topping is golden brown.
Let cool for 5-10 minutes before serving. There may be a little liquid in the bottom, that will become syrup-y as it sits. Enjoy with vanilla ice cream, if desired.
