Using a thin knife or a mandoline, cut each apple half into very thin slices, between 1/8 and 1/16-inch thick, keeping the halves together as they are sliced. Drizzle each of the cut halves with some of the lemon juice to keep them from discoloring.
Cut 6 pieces of parchment paper approximately 14 by 15 inches each. Working 1 at a time, arrange a piece of parchment on the counter in front of you so that one of the short sides is closest to you. Place 1 apple half in the center of each piece of parchment, and press down on the apple so that the slices are fanned out
in a line in line with the longer edges of the parchment, accordion-fashion, slightly flattened. Place a teaspoon of the butter and a teaspoon of the sugar on top of each apple half. Cut the vanilla bean into 6 equal pieces horizontally, then cut each piece in half lengthwise to reveal the inner seeds. Rub the exposed seeds of one of the pieces over the top of the apple slices and then place the bean on top of the apple slices. Bring the long edges of the parchment up so that they meet over the apple, and then fold together several times over the apple slices so that they are snugly enclosed. Repeat with the open ends of the parchment, tucking the edges underneath the apples after they are folded together 2 or 3 times, to make a neat package just slightly larger than the apple itself. Repeat with the remaining apples. Place the apple packages on a baking sheet and bake until the packages are puffed and lightly browned, about 15 minutes.
Remove from the heat and allow to cool for a few minutes before serving. To serve, place each apple packet on a dessert plate and either cut with scissors or allow guests to tear into their packets at the table so that the apple and vanilla aroma wafts up to greet them as they are opened. Serve a dollop of vanilla ice cream over each apple half at the table, if desired.