Preheat your oven to 350°F (180°C) and prepare a 9x13-inch baking pan by greasing it or lining it with parchment paper.

In a mixing bowl, combine all-purpose flour, granulated sugar, and a pinch of salt. Cut in the cold unsalted butter until the mixture resembles coarse crumbs. Press into the bottom of the pan and bake for about 15 minutes.

Mix brown sugar with reserved crumbs from the crust until clumpy, add oats or chopped nuts if desired. Set aside.

Combine granulated sugar, flour, ground cinnamon, and salt in a bowl. Add chopped peaches and stir in the heavy cream until thick and creamy.

Pour the peach filling over the warm crust and spread evenly. Sprinkle the streusel topping over the filling and bake for an additional 45 minutes.

Cool completely, then chill in the refrigerator for at least 2 hours before cutting into squares.