How to Make It
Whisk together 1 3/4 cups of the flour, 1/3 cup of the granulated sugar, and 1/2 teaspoon of the salt in a medium bowl. Using a pastry blender, cut 1/2 cup of the butter into flour mixture until well blended and mixture resembles peas. Stir in toasted pecans.
Stir together cream and 1/2 teaspoon of the vanilla in a small bowl; add cream mixture to flour mixture, stirring with a fork until mixture is combined but still crumbly. Chill until ready to use.
Preheat oven to 400°F. Gently toss together blackberries, blueberries, raspberries, strawberry halves, lemon zest, nutmeg, and remaining 5 tablespoons flour, 1/3 cup granulated sugar, 2 tablespoons butter, 1 teaspoon vanilla extract, and 1/4 teaspoon salt in a large bowl. Transfer to a lightly greased 10-inch cast-iron skillet. Place skillet in an aluminum foil-lined rimmed baking sheet.
Bake in preheated oven until bubbly around the edges, 20 to 22 minutes. Remove from oven. Crumble chilled streusel over hot filling, and sprinkle with turbinado sugar. Return to oven, and bake until golden, 25 to 28 more minutes. Serve warm.