Prepare the Casserole: Spray a 9x13-inch baking dish with non-stick cooking spray. Tear the loaf of bread into bite-size 1-inch pieces (you should have about 10 to 12 cups) and place in the baking dish in an even layer.
In a large bowl, whisk together the heavy cream, whole milk, eggs, brown sugar, vanilla, cinnamon, and salt. Evenly pour over the bread pieces in the baking dish. Using the back of a spoon, gently press down on the bread so that it is all submerged in the custard. Cover with plastic wrap and refrigerate overnight.
Make the Topping: In a medium bowl, whisk together the flour, brown sugar, cinnamon and salt. Add the butter and, using a pastry blender, fork, or your fingertips, work the butter into the flour mixture until only pea-sized pieces remain. Toss in the chopped pecans. Cover and refrigerate until ready to use.
Bake the Casserole: Preheat oven to 350 degrees F. Remove the casserole from the refrigerator and allow to sit at room temperature while the oven preheats.
Sprinkle the topping evenly over the casserole.
Bake until golden brown and puffed, approximately 40 to 50 minutes (an instant-read thermometer should register at least 165 degrees F when inserted into the center). Allow to rest for 5 minutes before serving. Top with powdered sugar and maple syrup for a traditional French toast flavor!
Bread: Do not use a French baguette, as it is too crusty for this recipe. If you choose to use a richer bread such as brioche or challah, I recommend letting the torn pieces sit out overnight (before assembling) to let it stale or to dry it in the oven, as these are softer breads that won’t hold up as well in the custard.
Custard: You can replace all of the heavy cream and whole milk with half-and-half if you’d like, or you can use all whole milk for a slightly lighter, albeit less rich, casserole. I do not recommend using low-fat milk in this recipe.
Nuts: You can substitute different nuts for the pecans in the topping, or omit them altogether.
Bake Immediately Option: While this is an ideal make-ahead recipe since it should be assembled the day before baking to allow the bread to absorb the custard, you CAN make it the same day, if desired. After pouring the custard over the bread, allow to sit at room temperature for at least 1 hour before baking. The inside will be less soft, but still delicious!
Freezing Instructions: This casserole can be baked, cooled completely, refrigerated for at least 2 hours, then frozen. To freeze, cover the pan tightly with foil and freeze for up to 3 months. Thaw overnight in the refrigerator, then reheat in a 350-degree oven, covered, until warmed through, about 15 minutes.