Overnight or at least one hour before wanting to bake, cut bread slices in half on the diagonal and place in a baking pan that is roughly 9 x 6 (see note). Mash raspberries and spread raspberries in between each slice of bread.
In a bowl, whisk together eggs, whole milk, maple syrup, vanilla, and lemon zest. Pour over the bread and push bread into the mixture so that all the bread is covered with the mixture. Cover and let sit until ready to bake.
Preheat oven to 375˚ and after french toast has sat and absorbed the majority of liquid, place in over to bake. Bake long enough so french toast is browning and has puffed up quite a bit.
While the french toast bakes, whip together (either using a hand mixer or whisk/hand) the heavy cream and maple syrup until whipped cream forms.
Serve french toast with a dollop of whipped cream, extra maple syrup, and extra fresh raspberries if desired.