Heat oven to 350°F. Grease and line a 9-inch pan with parchment.
Peel, core, and dice the apples. Toss with lemon juice and set aside.
Whisk flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a large bowl.
Whisk melted butter, oil, granulated sugar, brown sugar, eggs, vanilla, and buttermilk until smooth.
Pour wet mixture into dry. Stir gently until just combined; a few streaks are fine.
Fold in the apples and walnuts until evenly distributed.
Spread batter into the pan. Sprinkle turbinado sugar over the top for crunch.
Bake 40–45 minutes until golden and a toothpick comes out clean.
Cool 15 minutes in the pan. Slice and serve warm.