To make the skillet cake batter, whisk together the eggs, milk and 2 tablespoons butter, melted. Mix the flour, baking powder, sugar, and zest. Combine the flour with the liquids and allow batter to rest, refrigerated, for at least 30 minutes before cooking.
Peel, core, and cut the apples into 1/2-inch thick slices. In a separate pan, melt 2 tablespoons butter, add the apple slices and saute over medium heat until they turn a rich golden color, about 5 minutes. Reduce the heat to low and carefully add the maple syrup, Port, cinnamon, and lemon juice. Cook slowly until the apples are tender at the center of each slice, but not soft and mushy, about 2 minutes.
Heat a non-stick 7-inch omelet pan over low heat until hot. Add 1 or 2 teaspoons of butter, melted, using either a brush or a tiny ladle, to coat the surface lightly. As soon as the butter has started to bubble, lift the pan off the heat and pour 1/2 cup of batter into the pan, tilting to spread the batter evenly across the bottom. Return the pan to heat, spoon in 1/4 of the apples and cook on low until golden on the underside. When the top is bubbly, flip and brown the second side.
As each pancake is completed, place on an oven-proof platter set in a 200 degree oven. There should be enough batter to make at least 4 skillet cakes.
Serve each cake warm with a dollop of currant jelly and some clotted cream or creme fraiche.