1/2 teaspoon cornstarch
4 teaspoons fresh lemon juice
1 1/2 pounds Granny Smith apples (about 3 medium),
1 1/2 pounds Golden Delicious apples (about 3 medium),
2/3 cup granulated sugar
Pinch table salt
Pinch ground cinnamon
Pinch ground nutmeg
1 cup unbleached all-purpose flour
1/4 cup granulated sugar plus 1 tablespoon
1/4 cup packed brown sugar
1/8 teaspoon salt
2 teaspoons vanilla extract
6 tablespoons unsalted butter, cut into 6 pieces and very soft
1/2 cup sliced almonds or other nut
2. FOR THE FILLING: Meanwhile, combine cornstarch and lemon juice in large bowl until cornstarch is dissolved. Add apples, sugar, salt, cinnamon, and nutmeg; toss to combine. Transfer mixture to 8-inch-square glass baking dish. Cover tightly with foil and set aside.
3. FOR THE TOPPING: Combine flour, sugars (reserving 1 tablespoon granulated sugar), and salt in work bowl of food processor (or use pastry blender); drizzle vanilla over top. Pulse to combine, about five 1-second pulses. Add butter and half of almonds; process until mixture clumps together into large, crumbly balls, about 30 seconds, pausing halfway through to scrape down sides of work bowl. Sprinkle remaining almonds over mixture and combine with 2 quick pulses. Transfer mixture to parchment-lined baking sheet and spread into even layer (mixture should break up into roughly 1/2-inch chunks, with some smaller, loose bits). Place topping mixture on middle rack in oven and apple filling mixture on lower rack. Bake topping until chunks are lightly browned and firm, about 20 minutes. Remove both topping and apples from oven.
Remove foil from apple filling and gently stir. Grasping edges of parchment paper, slide topping over apples and spread into even layer with spatula, packing down lightly and breaking up any very large pieces. Sprinkle remaining tablespoon sugar over top and place on middle oven rack. Bake until well browned and fruit is tender and bubbling around edges, about 25 minutes. Cool on wire rack until warm, at least 15 minutes. Serve with ice cream, and drizzle caramel sauce over the top if desired.