Cut the day-old bread into roughly 1-inch cubes and set them in a large mixing bowl.
If the bread isn't fully dried out, spread the cubes on a baking sheet and toast them at 300°F for 10 minutes, then let them cool before continuing.
Peel and dice the peaches into ½-inch pieces and toss with the lemon juice in a small bowl.
Fold the peaches into the bowl of bread cubes and toss gently to distribute them evenly.
Grease the 9x13-inch baking dish with the extra tablespoon of butter and set it aside.
In a large bowl, whisk the eggs until combined and slightly foamy, then add the milk, cream, granulated sugar, and brown sugar, whisking until mostly dissolved.
Whisk in the melted butter, vanilla, cinnamon, nutmeg, and salt until smooth and pale yellow.
Pour the custard evenly over the bread and peaches, press down gently, and let soak at room temperature for 15 to 20 minutes.
While the bread soaks, preheat the oven to 350°F (177°C).
Pour the soaked mixture into the prepared baking dish, spreading evenly, and sprinkle with turbinado sugar.
Bake for 45 to 50 minutes, until deep golden brown and the center feels set; tent with foil if browning too quickly.
Remove from the oven and let rest for at least 10 minutes before serving.