Preheat the oven to 350 degrees F. Line 24 muffin cups with paper liners.
Whisk together the flour, baking soda, nutmeg and salt in a medium bowl. Set aside.
Beat the butter and sugar together, in a large bowl, using an electric mixer on medium-high speed until fluffy, about 3 minutes. Beat in the eggs, one at a time, and then the vanilla, beating until smooth.
Reduce the mixer speed to low and beat in half of the flour mixture until just combined. Add the sour cream, mixing to combine, and then the rest of the flour. Beat until well combined, taking care not to overmix. Stir in the chopped apples.
Divide the batter evenly among the prepared muffin cups, filling them almost halfway full. Bake until a toothpick inserted into the center of the cupcakes comes out clean and the tops spring back when pressed gently, about 20 minutes. Cool the cupcakes in the tin on a rack for 10 minutes then remove from the tin and cool completely. Transfer the cooled cupcakes to a baking sheet.
For the meringue frosting:
Preheat broiler on low.
Bring a few inches of water to a boil in a medium saucepan. Whisk together the egg whites, sugar, lemon juice and zest, cream of tartar and salt in a heat-safe bowl. Set the bowl over the saucepan, making sure the bottom of the bowl does not touch the water. Continue whisking until the mixture is hot to the touch and the sugar dissolves, about 2 minutes. Remove the bowl from the saucepan and beat, using an electric mixer at medium-high speed, until the meringue is cool and holds a stiff peak, about 7 minutes.
Using a spoon, offset spatula or piping bag, dollop, spread or pipe the meringue on top of each cupcake. Use the back of a spoon to create peaks and valleys in the frosting as desired.
Working in batches, place the cupcakes under the broiler and cook until the meringue is browned in spots, 1 to 2 minutes, watching carefully to avoid burning. Let cool.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
Copyright 2016 Television Food Network, G.P. All rights reserved.