If you’d like to experiment and try different flours, feel free. I do love using barley flour when baking with fruit, but if you’d prefer to substitute all-purpose or another flour (oat flour would be really nice), go right ahead: this is a rustic, forgiving recipe. You could also use any fruits that you like here. Next time I’m going to stir in some strawberry slices to accompany the rhubarb. To carry the cake through the seasons, it’d be wonderful with thinly sliced pears and cardamom in the fall, apples and quince in the winter and a heap of berries in the summer.
Preheat the oven to 375 F. Butter a 10-inch enamel or glass pie pan.
In a large mixing bowl, whisk together the flours, 1/2 cup of the sugar, ginger, cardamom, baking soda, baking powder and salt.
Work the butter into the flour mixture with your fingertips (or use a pastry blender if you prefer) until it’s incorporated and resembles small coarse peas. Add the beaten egg, 1 cup of buttermilk, vanilla extract and lemon zest. Using a fork or a wooden spoon, stir until it’s no longer crumbly and becomes a dough that you can gather together easily with your hands.
In a small bowl, combine the sliced rhubarb and remaining 1/4 cup sugar. Stir to combine.
Divide the dough into two and pat the first half into the prepared pan and pat down so it’s uniformly covering the bottom of the dish. Turn the rhubarb mixture out onto the top and spread evenly. It will be pretty heaped up–that’s okay. Take the second round of dough and pat it out into a rough circle on a clean work surface. Cover the rhubarb with the second round of dough as best you can (the top needn’t look perfect like a pie — it’s a rustic dish and you won’t have exactly enough dough to cover the rhubarb perfectly. If there are holes or open spots, no need to worry).
Using a pastry brush, brush the remaining 1 tablespoon of buttermilk across the top of the cake. Sprinkle the top with remaining 2 tablespoons of sugar.
Bake for 60-70 minutes, or until the top is golden and the rhubarb is bubbly and quite soft. Allow cake to cool for 15 minutes after removing it from the oven. Slice in wedges, much like a scone. Serve warm with a generous dollop of whole-milk plain yogurt. As with many whole-grain baked goods, this cake is best enjoyed the day it’s made. If you do have leftovers, wrap well and keep at room temperature for up to 1 additional day.