Heat oven to 350 degrees.

In a bowl, sift flour, salt, 3 tablespoons sugar. baking soda and lemon zest together.

With fingers, rub in butter until mixture is sandy. Beat 1 egg and add to flour mixture, add buttermilk and blend, then turn dough out onto a floured surface. Knead briefly until the dough comes together. Dough will be quite stiff and sticky. Divide in two. Roll out each piece to fit a 9-inch round baking dish. Line bottom of pan with one round, pinching together any tears.

Cover dough in dish with rhubarb. In a small bowl, mix together remaining sugar and 2 tablespoons of flour. Sprinkle evenly over rhubarb in dish. Place second pastry round on top and pinch edges together. Pinch together any holes. Beat remaining egg with 1 teaspoon water. Brush over dough.

Place a baking sheet in oven to catch any drips. Place baking dish on baking sheet and bake about 1 hour. If crust browns too soon, cover the cake with a piece of aluminum foil and continue baking.

Serve warm with whipped cream or vanilla ice cream, optional