1. Preheat oven to 400°.
2. Toss apples with lemon juice to coat evenly in a small bowl.
3. Spread melted butter in an even layer in the bottom of a 15- x 10-inch jellyroll pan or large ovenproof skillet; sprinkle with 1/4 cup sugar. Transfer apples to jellyroll pan, cut side down, reserving any lemon juice remaining in bowl.
4. Combine apple cider and vanilla with lemon juice left in the bowl; pour over the apples and into the pan. Sprinkle apples with remaining 1/4 cup sugar. Roast at 400° for 35 to 45 minutes or until apples are tender and juice mixture begins to caramelize, basting every 15 minutes. Serve warm in a pool of Buttermilk Custard Sauce and with the pan liquid spooned over the top.