Preheat your oven to 350°F (175°C).

Cut the day-old bread into 3/4-inch cubes and place in a large mixing bowl.

In a skillet, melt a couple of tablespoons of butter and sauté the chopped rhubarb for 3-5 minutes.

Stir the cooked rhubarb into the bowl of bread cubes.

In the same skillet, combine milk, brown sugar, and remaining butter; heat until sugar dissolves.

Pour the warm custard mixture over the bread and rhubarb, stirring gently to combine.

In a separate bowl, whisk together eggs, cinnamon, nutmeg, orange zest, and vanilla extract, then stir into the bread mixture.

Pour the entire mixture into your prepared baking dish and spread evenly.

Bake for about 40-50 minutes or until the center is set but slightly jiggly.

Remove from the oven and let cool for 5 minutes before serving warm.