We have two huge rhubarb plants in our garden, and in mid-late May when the bright red stalks are at their tenderest, I make this pie. Rhubarb pie fillings are often soft and runny. The rhubarb contains so much water, you either have to add too much thickener or coax the rhubarb to release its liquid before thickening it. What's the answer? To solve this problem, I mix the rhubarb with the sugar. Over time, the sugar draws out the excess juices. This takes place overnight, while you're sleeping, and by the next morning, the rhubarb's ready to be put into the pie. After baking and cooling, you can slice into this pie and the filling will stay put. Be sure to make this pie when rhubarb is tender and bursting with flavor. The pastry is half butter and half lard. Cider vinegar helps prevent excessive gluten formation. You will not taste it at all. A serving of fresh rhubarb pie. See how flaky the pastry is? And the filling is moist and slightly tart and stays put.