Step 1: Prepare Your Baking Dish. Lightly grease a 9x13 inch baking dish with butter or non-stick spray.
Step 2: Slice and Arrange the Bread. Cut your chosen bread into 1-inch thick slices or 1-inch cubes. Arrange them in a single layer in the prepared baking dish, overlapping slightly if using slices, or scattering evenly if cubing.
Step 3: Whisk the Custard. In a large bowl, whisk together the eggs, whole milk, heavy cream (if using), 1/2 cup brown sugar, vanilla extract, 1 teaspoon ground cinnamon, nutmeg, and a pinch of salt until well combined and smooth.
Step 4: Soak the Bread. Slowly and evenly pour the egg mixture over the bread in the baking dish, ensuring all pieces are thoroughly saturated. Gently press down on the bread to help it absorb the liquid.
Step 5: Chill Overnight (or at least 4 hours). Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or ideally, overnight. This allows the bread to fully absorb the custard.
Step 6: Prepare the Pecan Streusel Topping. In a medium bowl, combine the all-purpose flour, 1/2 cup brown sugar, 1 teaspoon ground cinnamon, and cold butter cut into small pieces. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in the chopped pecans. Set aside.
Step 7: Bake to Golden Perfection. Preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and uncover. Sprinkle the pecan streusel topping evenly over the top of the casserole.
Step 8: Bake for 45-55 minutes, or until the casserole is golden brown, puffed up, and the center is set (a knife inserted into the center should come out clean). If the topping browns too quickly, loosely tent with aluminum foil.
Step 9: Serve and Enjoy! Let the casserole rest for 5-10 minutes after removing it from the oven before serving. Serve warm with maple syrup, powdered sugar, or fresh berries.