Preheat your oven to 350°F (180°C) and lightly grease a 9x13-inch baking pan with butter or cooking spray.

In a mixing bowl, combine all-purpose flour, granulated sugar, and salt. Cut in the cold unsalted butter until mixture resembles coarse crumbs. Press into the bottom of the greased pan.

Bake in preheated oven for about 15 minutes, or until the crust turns lightly golden.

Mix together the remaining crumb mixture with brown sugar in a separate bowl. If desired, stir in oats or chopped nuts.

In a large bowl, combine the prepared peaches, granulated sugar, flour, ground cinnamon, and heavy cream. Mix well.

Pour the peach filling evenly over the warm crust and sprinkle the streusel topping over the peach layer.

Return to oven and bake for an additional 45 minutes, or until topping is golden brown and juices are bubbling.

Remove from oven and let cool completely in the pan on a wire rack. Refrigerate for at least 2 hours before slicing.

Once chilled, cut into squares or rectangles and serve cold or warm with whipped cream or vanilla ice cream.