Grease a 9x13 inch baking dish with butter.
Place the cubed French bread in an even layer in the baking dish.
In a large mixing bowl, whisk together the eggs, milk, heavy cream, sugar, vanilla extract, ground cinnamon, and ground nutmeg until well combined.
Pour the egg mixture evenly over the bread cubes, pressing down gently to ensure all the bread is soaked.
Cover the baking dish with plastic wrap or aluminum foil and refrigerate overnight (or for at least 4 hours).
When ready to bake, preheat the oven to 350°F (175°C).
Remove the cover and bake the French toast uncovered for about 45 minutes, or until the top is golden brown and puffed.
Let it cool for a few minutes before serving. Cut into squares and serve warm with optional toppings like maple syrup, powdered sugar, or fresh fruit.