Preheat the oven to 180C / 350°F Grease a 20cm / 8in cake pan, preferably loose-bottomed or springform. If you don't have one, use a regular cake pan but grease very well, line the bottom with a circle or parchment paper, and grease again.
Trim the rhubarb and chop into pieces a couple of inches long. Put in a baking dish with the sugar and water and bake for 20-30 minutes, until the rhubarb is soft but still retains its shape. Drain the fruit and set aside you can reserve the juice to serve with the cake if you wish.
Put the cornmeal, flour, baking powder, cinnamon and sugar in a large bowl, or in the bowl of a food processor or a stand mixer with the paddle attachment. Add the grated zest and the butter. If using your hands, rub the butter in as if making pastry - until the mixture resembles large crumbs. If using a food processor you just need to blitz for a few seconds, and if using a stand mixer, mix on low to medium speed for about 30 seconds.
Break the egg into a small bowl and mix with the milk. Now add slowly to the crumble mix, stopping as soon as everything has come together to form a soft, sticky dough. You may not need all the liquid, or you may need a little more milk to get the right consistency.
Press about two thirds of the crumble mixture into the bottom of the cake pan, pushing a couple of centimetres up the sides of the pan. Place the rhubarb on top, being careful to leave a small rim around the edges uncovered. Crumble the rest of the mixture over the fruit in fat lumps, using your fingers - don't worry if the fruit isn't all covered. Scatter over the brown sugar.
Bake for 40-45 minutes. Cool the cake a little before removing from the pan.