Preheat the oven to 350F. Grease a 9×9 baking pan and set aside.
In a medium bowl mix together the butter, 1 tsp Italian seasoning, and grated garlic. Spread the butter evenly onto the cut side of the baguette. Using a serrated knife cut the baguette into 1 ½ inch cubes and place cubes butter side up, on a large baking sheet lined with parchment paper. Bake for 10-12 minutes. Allow the pieces to cool for 10 minutes.
Meanwhile, whisk together the eggs, whole milk, remaining Italian seasoning, salt & pepper in a large bowl, until no streaks of egg whites remain. Fold 1 cup of shredded mozzarella cheese, pepperoni, and optional add-ins into the egg mixture.
After the toasted garlic bread cubes have cooled for 10 minutes, fold them into the egg mixture. Pour the mixture into the greased baking pan and spread the bread cubes out evenly and press them down gently into the egg mixture. Sprinkle the remaining 1 cup of mozzarella cheese on top, followed by the marinara sauce.
You can bake it right away, but for best results, cover and refrigerate for 4 hrs, or up to overnight. If chilling, remove the pan from the fridge and allow it to sit on the counter for 30 minutes prior to baking. Bake at 350F for 45 minutes. Allow to cool for 10 minutes prior to serving.
Slice and serve with additional warm marinara sauce.