Lightly grease a 9x13-inch baking dish.
Spread half of the bread cubes evenly in the dish. Sprinkle half of the mixed berries over the bread. Repeat with the remaining bread and berries.
In a large bowl, whisk together the eggs, milk, heavy cream, granulated sugar, brown sugar, vanilla extract, cinnamon, and salt until fully combined.
Pour the custard mixture evenly over the bread and berries, pressing down gently to ensure the bread absorbs the liquid.
Cover the dish with plastic wrap and refrigerate for at least 3 hours, or overnight for best results.
In a small bowl, mix the flour, brown sugar, cinnamon, and cold butter using a pastry cutter or your fingers until crumbly. Stir in the sliced almonds or pecans, if using.
Preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and sprinkle the topping evenly over it.
Bake uncovered for 40-50 minutes, or until the top is golden and the custard is set.
Let the casserole cool for 5-10 minutes before serving. Garnish with powdered sugar, maple syrup, and additional fresh berries as desired.