Preheat oven to 350 degrees. In a large bowl, combine the rhubarb with 2 cups of the sugar, 4 tablespoons of the flour, nutmeg and cardamom. Stir well and let sit while you prepare the cake.
In a small bowl combine the remaining 1 1/3 cups flour with the baking powder, baking soda and salt, and set aside. In the bowl of an electric mixer cream the butter (or coconut oil) and 1 cup sugar for at least 3 minutes, until light and fluffy. Add the eggs, one at a time, waiting until each is well incorporated before adding the next egg. Add the vanilla and lemon zest and juice and mix briefly. Last add the dry flour mixture slowly while mixing on low speed, and mix until just combined.
Pour the rhubarb mixture into a 3.5 quart dutch oven, large soufflé dish, or casserole dish. Spread the cake mixture evenly over the top of the rhubarb. Bake for approximately 90 minutes, until the top is golden and a bit crusty, and the rhubarb caramel is golden and bubbly at the edges of the cake. Let cool at least until you can touch the dish with your bare hands, then run a knife around the edges of the cake to loosen, and turn it over onto a plate.
