01 - Preheat the oven to 350°F (175°C). Grease and line a 9-inch springform or deep cake pan with parchment paper.
02 - In a skillet, heat olive oil over medium heat. Sauté chopped onions and diced celery until softened, approximately 5 minutes. Incorporate minced garlic, dried thyme, and dried sage; cook for an additional minute. Stir in chopped spinach until just wilted. Remove from heat and combine with shredded or diced turkey, salt, and pepper. Allow to cool slightly.
03 - In a large mixing bowl, whisk together all-purpose flour, baking powder, baking soda, and salt.
04 - In a separate bowl, whisk together large eggs, full-fat sour cream, whole milk, and melted unsalted butter until smooth. Gently fold in grated Parmesan cheese.
05 - Add the dry ingredients to the wet ingredients, stirring gently until just combined. Fold in the cooked turkey and vegetable mixture until evenly distributed throughout the batter.
06 - Pour the prepared batter into the prepared cake pan, smoothing the top surface.
07 - In a bowl, combine the grated sweet potato, chopped pecans, rolled oats, brown sugar, cinnamon, nutmeg, and a pinch of salt. Add the cold, cubed unsalted butter and rub it into the mixture with your fingertips until coarse crumbs form.
08 - Evenly sprinkle the sweet potato crumble mixture over the cake batter. Bake for 60 to 70 minutes, or until a skewer inserted into the center comes out clean and the topping is golden and crisp.
09 - Allow the cake to cool in the pan for 15 minutes before releasing the sides and transferring it to a wire rack to cool completely. Serve warm or at room temperature.