Preheat the oven to 375°. In a 10 1/2-inch nonstick ovenproof skillet, melt the butter. Add the apples, 1 tablespoon of the sugar and the lemon juice and cook over moderately high heat, stirring occasionally, until the apples are golden, about 6 minutes. Add the 1/4 cup of maple syrup and simmer over low heat until thickened, about 1 minute. Spread the apples in an even layer and remove the pan from the heat.
In a medium bowl, whisk the flour with the baking powder and salt. In a measuring cup, whisk the egg yolks with the milk and the remaining 2 tablespoons of sugar. Whisk the liquid into the dry ingredients.
In a medium bowl, using a handheld electric mixer, beat the egg whites at medium speed until firm peaks form, about 2 minutes. Fold the beaten whites into the batter and scrape it over the apples; spread the batter to the edge.
Bake the pancake in the upper third of the oven for about 20 minutes, until it is golden, puffed and set. Let the pancake cool for 5 minutes. Run a knife around the edge to loosen the pancake, then invert it onto a serving plate. Replace any apples that may have stuck to the pan. Cut the pancake into wedges and serve at once with the warmed maple syrup.