Rub or cut (using a pastry cutter or two knives) the butter into the flour mixture, until it all looks like small peas.
Beat together egg, cold water and lime juice.
Gradually add liquid mixture to flour mixture, stirring gently with a fork. You may need to add the 6th tablespoon of cold water. Do not overwork the dough or your crust will be tough.
Shape mixture into two balls, wrap each in wax paper and refrigerate for 20 minutes.
Combine all the filling ingredients, in a large bowl, in the order listed. Stir well and let sit for 10-15 minutes.
Just before rolling out the pie crust dough, start to pre-heat the oven to 400°F.
Flatten and roll out one ball of pie crust and use it to line a 9 or 10-inch pie pan. I use a 10-inch pan. For ease of handling, roll out the crust between two sheets of floured wax paper; also flour the rolling pin.
Pour pie filling into crust.
Flatten and roll out second ball of pie crust. Use it to cover the filling, then crimp around the edges.
Cut air slits.
Brush top crust with finishing lime juice and sprinkle with sugar.
Put in 400°F oven for 40 minutes or until rhubarb is tender.
Let rest for at least 60 minutes (one hour) before slicing.