Butter a 9×13-inch baking dish and arrange French bread slices in a single layer.
In a large bowl, whisk eggs, half-and-half, milk, vanilla, sugar, cinnamon, and nutmeg until smooth. Pour over bread, pressing lightly to soak. Cover and refrigerate overnight.
The next morning, preheat oven to 350°F (175°C). Uncover, dot with butter, sprinkle with sugar, and bake 35–40 minutes until puffed and golden.
Cool for 2–3 minutes, then sprinkle with sugar and broil 1–2 minutes (or torch) to caramelize.
Serve warm with syrup, berries, or whipped cream.