Begin by greasing a 9×13 inch baking dish and set it aside.
Cut the day-old French bread or brioche into 1-inch cubes and arrange them in a single layer in the prepared baking dish.
In a mixing bowl, whisk together 2 cups of milk, 1/2 cup of granulated sugar, 1/2 cup of pumpkin puree, 4 large eggs, 1 tablespoon of vanilla extract, and 1 tablespoon of pumpkin pie spice (or cinnamon) until well combined.
Pour the mixture evenly over the bread cubes, ensuring all the bread is soaked. Gently press down to guarantee even soaking.
In a separate bowl, prepare the streusel topping by combining 1/2 cup of light brown sugar, 1/2 cup of all-purpose flour, diced cold butter, chopped pecans, and 1 teaspoon of cinnamon. Use a pastry cutter or your fingers to blend the mixture into a crumbly texture.
Sprinkle the streusel topping evenly over the bread mixture in the baking dish.
Cover the baking dish with foil and refrigerate for at least 2 hours or overnight. This allows the bread to absorb the liquid and flavors.
When ready to bake, preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius).
Bake the French Toast Casserole, covered with foil, for 30 minutes.
Remove the foil and bake for an additional 15-20 minutes or until the top is golden brown and the casserole is set.
Let it cool for a few minutes before serving. Drizzle with maple syrup or sprinkle with powdered sugar if desired. Now, enjoy your delicious Pumpkin French Toast!