From lemons, finely grate 1 1/2 teaspoons peel and squeeze 2 tablespoons juice. Reserve 2 Granny Smiths and 1 Golden Delicious apple for top. Slice remaining apples. In large skillet, melt 4 tablespoons butter over medium-high heat. Add sliced apples and cook 5 minutes. Cover, reduce heat to medium, and cook 15 minutes or until very tender. Stir in lemon peel and juice, 1/2 cup sugar, vanilla and nutmeg. Cook, stirring frequently, 25 to 30 minutes until puree is very thick and has reduced to 2 1/4 cups. Cool to room temperature.

Meanwhile, preheat oven to 375° F.

Thinly slice remaining apples. Spoon purée into baked, cooled tart shell and spread evenly. Arrange apple slices, overlapping in concentric circles, on purée. Melt remaining 2 tablespoons butter. Brush apples with butter and sprinkle with remaining 2 tablespoons sugar. bake 45-50 minutes, until apples are tender when pierced with a knife. Cool tart on a wire rack 10 minutes. Remove side of pan and cool completely. When cool, Brush apple slices with sieved preserves.