Preheat oven to 375°F. Discard any leaves on the stalks of rhubarb and, if needed, peel or pull any strings off the stalks. Chop into small cubes and place in a buttered baking dish (large enough to hold the bread mixture with room to stir in the milk and egg mixture). Add the candied ginger, sugar, salt, lemon zest and juice and stir thoroughly. Then add the bread cubes and toss to combine. Lightly beat the egg with the milk and pour the mixture over the fruit and bread cube mixture. Cut the butter into small pieces and add to the top.