1. Position a rack in center of oven. Preheat to 350° F. Grease a nonstick (9-inch) round cake pan.
2. In a small saucepan over low heat, melt 3 tablespoons butter. Whisk in brown sugar, cinnamon, vanilla and salt until blended, about 1 to 2 minutes. Pour sauce into prepared pan, and arrange apple slices on top. 
3. In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon and nutmeg. In another large bowl, beat together butter and sugar with a hand mixer until softened. Add eggs one at a time. Add vanilla.
4. Add half of the dry ingredients to the wet ingredients, beating until combined. Add mascarpone; mix until fully incorporated. Add remaining dry ingredients. Stir until just combined. 
5. Pour batter evenly over apples, smoothing the top. Bake for 30 minutes, or until cake is golden and a toothpick comes out clean. Let cool for 10 minutes before inverting warm cake onto a serving plate.

Calories: 318, Fat: 13 g (8 g Saturated Fat), Cholesterol: 75 mg, Sodium: 216 mg, Carbohydrates: 47 g, Fiber: 2 g, Protein: 5 g.