Step 1: Grease a 9x13 inch baking dish. Arrange the bread cubes evenly in the prepared dish. If your bread is fresh, you can dry it out by toasting in a 300°F (150°C) oven for 10-15 minutes.
Step 2: In a large bowl, whisk together the eggs, milk, granulated sugar, vanilla extract, cinnamon, nutmeg, and salt until well combined and frothy. Pour this mixture evenly over the bread cubes, ensuring all pieces are moistened. Gently press down on the bread to help it absorb the liquid.
Step 3: Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight. This allows the bread to fully soak up the custard, making for a truly custardy french toast casserole recipe.
Step 4: Prepare the streusel topping: In a medium bowl, combine the brown sugar, flour, softened butter, and chopped pecans (if using). Use your fingers or a pastry blender to mix until the mixture is crumbly.
Step 5: When ready to bake, preheat your oven to 375°F (190°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 30 minutes.
Step 6: Evenly sprinkle the streusel topping over the soaked bread in the dish. Bake uncovered for 45-55 minutes, or until the casserole is golden brown, puffed, and the center is set (a knife inserted into the center should come out mostly clean). If the topping browns too quickly, loosely tent with foil.
Step 7: Let the french toast casserole recipe cool for 5-10 minutes before serving. Dust with powdered sugar, drizzle with warm maple syrup, and garnish with fresh berries as desired. Enjoy this delightful french toast casserole recipe!