Spray a 9x13 inch baking dish with cooking spray and set aside.
Slice the French bread into large 1.5 to 2 inch slices and cut each slice into 1.5 to 2 inch cubes.
Place the bread cubes in the prepared pan. (It will be very full.)
Mix together the milk, cream, sugar, eggs, egg yolks, and vanilla until well combined.
Carefully pour the milk mixture over the bread cubes. Cover tightly with plastic wrap and store in the fridge for 1 to 24 hours until you are ready to bake it.
When you are ready to bake the casserole, remove it from the fridge and remove the plastic wrap. Preheat the oven to 375 degrees F and let the casserole rest at room temperature while the oven is heating.
When the oven is fully heated, place the casserole in the oven and bake for 50-65 minutes or until a knife inserted into the center comes out clean.
While the French toast casserole is baking, make the raspberry sauce. In a small bowl combine the sugar and cornstarch and stir together with a fork until there are no lumps.
In a medium sauce pan add the raspberries and cook over medium heat until the berries start to release some of their juice, stirring occasionally, about 3 minutes.
Stir in the sugar mixture. Increase the heat to medium-high and cook, stirring constantly until the mixture comes to a boil and boils for one minute. Remove from the heat.
When the casserole is cooked through remove it from the oven and dish it on plates. Serve it with the raspberry sauce over the top.
Store leftover sauce and casserole separate in the fridge.
Notes
The French bread can be store-bought or homemade. You need about 16 ounces of bread or 10 cups of large cubed pieces. I used my mom's homemade French bread recipe and it was perfect.
You can reduce the sugar in the French toast bake portion by 1/2 to 3/4 of a cup with no issues if you like a less sweet French toast, I normally make it with less sugar but it is really good with all of it.
You have some flexibility when it comes to how long this dish rests. I have let it rest as short as 1 hour and as long as overnight. It does need a little time to rest so that the bread can soak up the milk mixture but it doesn't have to be a full overnight rest. You can make it work for your schedule.
Use the leftover egg whites to make meringue cookies or add them to additional eggs for scrambled eggs.