Use fresh rhubarb when possible and cut it into small pieces for even baking. If the rhubarb is very juicy, toss it with 1 teaspoon of flour before folding into the batter. Greek yogurt can be used for a richer texture; add 1 tablespoon milk if it is very thick. Cover loosely with foil if the pistachios brown too quickly. Store covered at room temperature for 1 day or refrigerate for up to 4 days. Freeze individual slices for up to 2 months.