1Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
2In a medium bowl, mix together the flour, 1/2 cup of sugar, baking powder, baking soda and salt. Mix in 1/4 cup of butter with a fork, or rub between your fingers until it is in small pieces. Stir in the buttermilk just until blended. Spread evenly in the bottom of the prepared pan.
3Make a layer of rhubarb over the batter in the dish, then sprinkle 1 cup of sugar over it. Dot with remaining butter. Sprinkle with nutmeg, then pour boiling water over the whole thing.
4Bake for 45 minutes in the preheated oven, until the batter is cooked through and rhubarb is tender. Serve warm or cold.