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 thoroughly. Cut in the cold unsalted butter until the mixture resembles coarse crumbs.

Press the crumbly mixture firmly into the bottom of the greased pan and bake for about 15 minutes, or until lightly golden.

Mix together the remaining crumb mixture with brown sugar in a separate bowl. Stir in oats or chopped nuts if desired.

Combine the prepared peaches, granulated sugar, flour, ground cinnamon, and heavy cream in a large bowl.

Spread the peach filling evenly over the warm crust and sprinkle the streusel topping generously.

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

Let cool completely in the pan on a wire rack. Refrigerate for at least 2 hours.

Cut into squares or rectangles and serve cold or warm.