Preheat your oven to 350°F. Prepare a 9"x13" baking pan by coating it with non-stick cooking spray. Slice the French bread into cubes approximately 1 to 2 inches in size and place them in a large mixing bowl. In a separate bowl, vigorously whisk together the milk, cream, eggs, sugar, vanilla, cinnamon, and nutmeg until homogenous. Gently pour this custard over the bread cubes, stir carefully to distribute the mixture evenly, and allow it to rest for 20 minutes so the bread can absorb the liquid thoroughly. Transfer the soaked bread mixture into the prepared baking dish and bake for 50 to 60 minutes, or until the center is firmly set.

To prepare the sauce, combine the sugar, flour, and cream in a small saucepan, whisking until smooth. Introduce the butter and heat over medium flame until the butter has fully melted and the mixture comes to a gentle boil. Continue whisking until the sauce slightly thickens, then remove from heat and incorporate the vanilla extract.