In the bowl of a food processor, pulse the flour, sugar and salt to combine. Cut the cold butter into cubes. Add the butter to the food processor and process until they are the size of peas.
With the motor running on low, pour 5 Tbsp. very cold water through the feed tube until a crumbly dough forms.
Remove the dough and press it into a disc. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.
Make the Filling
Melt the butter in a small saucepan over low heat. Add the sugar. Stir continuously until the mixture becomes an amber color. Don't worry if it gets clumpy.
Stir in the apple slices, cinnamon and nutmeg. Cook, stirring occasionally, until the apples are softened.
In a small bowl, combine the cornstarch with 1 1/2 Tbsp. of water. Pour the mixture into the apples and stir until thickened. Remove from heat.
Form the Crostada
Preheat the oven to 350 degrees.
Using a rolling pin, roll dough into a large circle on a lightly floured surface. Transfer it to a baking sheet lined with parchment paper.
Spread the apple mixture in the center of the dough, leaving a perimeter of dough about 2 inches.
Fold and gather the dough in towards the center, slightly overlapping other pieces of dough and overlapping the apple mixture by 1/2 inch.
Brush the dough with heavy cream. Sprinkle it with turbinado sugar. Bake 50 minutes, until golden brown. Allow to cool on the baking sheet before transferring it to a serving plate.