Preheat oven to 350F and grease and line a 9x5-inch loaf pan with a strip of parchment paper.
Combine ½ cup light or dark brown sugar and 2 teaspoons ground cinnamon in a small bowl and stir to combine. Set aside. Have the 1 ½ cups apples ready.
Cream the ½ cup unsalted butter and 1 cup granulated sugar in a large bowl using a hand or stand mixer for about 2 minutes. Beat in the 2 large eggs until well combined.
On low speed, beat in the 1 ½ cups oat flour, ¼ cup almond flour, 1 ½ teaspoons baking powder, and ½ teaspoon fine sea salt, then beat in the ½ cup whole milk until a smooth batter forms.
Add half the batter to the prepared pan, top with half of the chopped apples, and sprinkle over half of the brown sugar mixture. Repeat this layering one more time using the remaining ingredients. Rest the batter for 15-20 minutes.
Place in the oven and bake for 1 hour, or until an inserted toothpick comes out clean. Cool 20 minutes in the pan set on a rack, then gently turn out onto the rack to cool completely before slicing.
Notes
Cool the bread thoroughly. It's a bit delicate when warm, so my advice is to let the bread completely cool to room temperature before slicing.
Use all-purpose gluten-free flour. If you prefer, swap the total amount of flour (use 1 ¾ cup/210 grams) for a gluten-free flour blend.
Add walnuts. When I'm in the mood for a bit of crunch, I add ½ cup of chopped walnuts to the batter!
Make it dairy-free. Substitute softened coconut oil for the butter and a non-dairy milk you like for the milk.
Another spice. Cinnamon is a classic, but ground ginger or cardamom (or a combination) is lovely!