In a large mixing bowl, whisk together the whole milk, all-purpose flour, eggs, vanilla extract, ground cinnamon, ground nutmeg, salt, and sugar until smooth and well combined.
Take six slices of bread and spread 2 tablespoons of peanut butter on each slice. Top with the remaining slices of bread to create six sandwiches.
Carefully dip each sandwich into the custard mixture, allowing them to soak for a few seconds on each side.
Heat a griddle or frying pan over medium heat and lightly grease it. Cook the soaked sandwiches for about 4-5 minutes on each side until golden brown and crispy.
Transfer the cooked French toast to a serving plate. Drizzle with maple syrup and dust with powdered sugar if desired. Serve warm.