Preheat the oven to 325 degrees F (165 degrees C).
Place bread cubes into a buttered 2 quart casserole dish. Combine the milk and butter in a saucepan, and heat just to the boiling point. Pour over the bread cubes, and let stand for 15 minutes. In a medium bowl, whisk together the eggs, sugar, cinnamon and salt. Stir in rhubarb. Pour over the soaked bread, and stir gently until evenly blended. Sprinkle walnuts over the top.
Bake for 50 minutes in the preheated oven, until nicely browned on the top. Let stand for 10 minutes before serving.
