1. Preheat oven to 350°. Bake pecans in a single layer in a shallow pan 8 to 10 minutes or until toasted and fragrant, stirring after 5 minutes. Increase oven temperature to 375°.
2. Toss apple slices with lemon juice, 1 tsp. vanilla extract, and 1/2 tsp. cinnamon.
3. Melt 1/4 cup butter in a 10-inch cast-iron skillet over low heat. Remove from heat; stir in brandy. Sprinkle with brown sugar.
4. Sprinkle pecans over brown sugar mixture. Arrange apples in 2 concentric circles over pecans.
5. Beat 1/2 cup granulated sugar and remaining 1/4 cup butter at medium speed with an electric mixer until blended. Add egg yolks, 1 at a time, beating just until blended after each addition. Add milk, sour cream, and remaining 1 tsp. vanilla, beating just until blended.
6. Whisk together baking mix, nutmeg, and remaining 1/4 tsp. cinnamon in a medium bowl. Add nutmeg mixture to butter mixture, beating just until blended.
7. Beat egg whites in a large bowl at high speed until soft peaks form. Gradually beat in remaining 1/4 cup granulated sugar until stiff peaks form. Fold into batter. Spread batter over apples in skillet.
8. Bake at 375° for 50 to 54 minutes or until a wooden pick inserted halfway into center of cake comes out clean. Cool in skillet on a wire rack 10 minutes. Carefully run a knife around edge of cake to loosen. Invert cake onto a serving plate, replacing any topping that sticks to skillet on cake.
Note: We tested with Bisquick Original All-Purpose Baking Mix.
To get a true reading of doneness, insert the wooden pick only halfway through the cake when testing.