Generously grease a 9x13 inch (3-quart) baking dish with butter. Arrange the cubed bread in a single, even layer in the dish.

In a large mixing bowl, whisk together the 8 large eggs until the yolks and whites are fully combined.

Add the whole milk, heavy cream, granulated sugar, 1/4 cup brown sugar, vanilla extract, 1 1/2 tsp cinnamon, nutmeg, and salt to the eggs. Whisk thoroughly until the mixture is smooth and the sugars have dissolved.

Slowly and evenly pour the egg custard mixture over the bread cubes in the baking dish. Gently press down on the bread with a spatula to ensure all pieces are saturated.

Cover the dish tightly with plastic wrap or a lid and refrigerate for at least 4 hours, or preferably overnight. This allows the bread to fully absorb the rich custard.

When ready to bake, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 20-30 minutes while the oven heats.

While the casserole rests, prepare the streusel topping. In a small bowl, combine the all-purpose flour, 1/2 cup brown sugar, and 1 tsp cinnamon.

Add the cold, cubed butter to the flour mixture. Using your fingertips, a pastry blender, or two knives, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with pea-sized pieces of butter remaining.

Stir the chopped pecans into the streusel mixture. Uncover the casserole and sprinkle the topping evenly over the surface.

Bake for 45 to 55 minutes, or until the top is golden brown and crispy, and the center is set (a knife inserted into the center should come out clean). If the top begins to brown too quickly, you can loosely tent it with aluminum foil for the last 10-15 minutes of baking.

Remove from the oven and let the French toast casserole cool for at least 10 minutes before slicing and serving. This allows it to set up properly.

Serve warm with a dusting of powdered sugar, a drizzle of maple syrup, and fresh berries, if desired.