Prep. Spray a 10-12 cup standard bundt pan with pan spray, or coat with butter, then dust with about 2 tablespoons of flour, tapping around to fully coat all sides. Tap out any excess flour and set aside. Preheat the oven to 350°F. Peel and finely dice the apples. Toast the walnuts, if using, in the preheated oven for about 5-7 minutes until fragrant, then coarsely chop.
Mix the dry ingredients. Whisk together the flour, baking powder, baking soda, salt, and spices.
Mix the wet ingredients. In a large bowl, whisk together the sugars, oil, eggs, and vanilla. Stir in the applesauce and Greek yogurt.
Combine. Mix in the dry ingredients until just a few flour streaks remain.
Add the apples. Gently fold in the chopped apples (and walnuts if using) until just combined.
Bake. Pour the batter evenly into the Bundt pan and bake at 350°F for 45-50 minutes, until the cake springs back or a toothpick comes out with just a few moist crumbs.
Cool. Let cool for about 10-15 minutes, then flip out and allow to cool fully. Drizzle the cake with the caramel and sprinkle with flaky sea salt. Slice and serve with more caramel sauce.