In a bowl combine flour, salt and sugar. With a fork or a pastry cutter, work half of the butter until it resembles coarse meal. Work the remaining butter into the dough.
Drizzle a little water at a time, mixing and tossing between each. Stop adding water when it still has a little bit of dry flour in the bottom of the bowl. Do not over mix the dough or it will become hard.
Divide the dough in two, form a disk and wrap in plastic wrap. Place in the refrigerator to rest for 30 minutes.
Combine rhubarb and strawberries in a large bowl. In a separate bowl mix in sugar and tapioca. Fold tapioca into the strawberries and rhubarb. Allow to sit for 15 minutes.
When ready to cook, start the Traeger grill on Smoke with the lid open until the fire is established (4 to 5 minutes). Set the temperature to 375 degrees F and preheat, lid closed, for 10 to 15 minutes.
Using a rolling pin, roll one of the dough disks into a circle large enough for a 10-inch pie dish. Lay dough on the dish and cut the excess around the pan. Prick bottom with a fork.
Lay a piece of parchment paper on top of the pie dough. On top of the parchment paper place some dry rice or beans to keep the parchment paper down. Bake on the Traeger for 10 to 15 minutes.
Take the pie off the grill and discard the parchment paper and rice/beans. Fill the pie with strawberry rhubarb filling. Dot with butter all around top of filling.
Roll out the 2nd pie dough and place on top of the filling. Cut off excess dough and pinch sides to close. With a knife make a few slits on top.
Brush the top with egg wash (1 egg and a little water mixed together). Sprinkle sugar on top.
Take pie to grill and bake for about 45 minutes. Pie shell should be cooked in the center and should have a golden brown color. Enjoy!