Preheat your oven to 350°F (175°C).

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

Melt a tablespoon of butter in a medium skillet over medium heat, add chopped rhubarb, and cook for 3-5 minutes until softened.

Add milk, remaining butter, and brown sugar to the skillet, heat until sugar dissolves while stirring gently.

Pour the custard over the bowl of bread cubes and cooked rhubarb. Stir gently and let rest for a few minutes.

Whisk together eggs, cinnamon, nutmeg, orange zest, and vanilla in a separate bowl, then stir into the bread mixture.

Transfer mixture to a greased 9x9-inch baking dish and bake for 40-50 minutes until golden brown on top.

Let cool for 5 minutes before slicing and serving, optionally with whipped cream or ice cream.