Preheat the oven to 350°F (175°C).

Grease a 9x13 inch baking dish with butter or cooking spray.

In a large mixing bowl, combine the flour, oats, brown sugar, granulated sugar, cinnamon, baking powder, and salt.

In another bowl, whisk together the melted butter, honey, eggs, and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients and stir gently until just combined.

Gently fold in the diced apples and walnuts, if using.

Transfer the batter to the prepared baking dish, spreading it evenly.

Bake in the preheated oven for 30 minutes or until a toothpick inserted in the center comes out clean.

Allow it to cool in the dish for a few minutes before slicing.