Preheat the oven to 350 degrees F (175 degrees C). Grease an 8-inch square baking dish.

Place toasted bread into the prepared baking dish.

Heat milk in a saucepan over medium-low until bubbles form around the sides. Remove from heat; add butter, stirring until melted. Pour milk mixture over bread; stir until bread evenly coated, then let stand 15 minutes.

Stir rhubarb, white sugar, eggs, cinnamon, and salt together in a bowl; pour into bread mixture and stir. Sprinkle brown sugar over top of rhubarb mixture.

Bake in the preheated oven until golden and set, 45 to 50 minutes.