Based on the Strawberry Rhubarb Pie Recipe from Joy of Cooking.
In a big bowl, combine the cut rhubarb and strawberries with the sugar and mix well.
Cover and let this mixture sit overnight in the refrigerator. If you like to let your pie dough sit overnight, prepare your favorite double pie crust recipe now. I have had success with King Arthur Flour's Classic Double Pie Crust Recipe.
Take the mixture out of the fridge and mix. Set a strainer or colander over a medium-sized bowl (can hold 4 cups or more) and strain the mixture.
Preheat the oven to 425°F and place a rack in the bottom level. If you haven't already made your double crust pie dough yet, do so now.
Pour the strained liquid into a measuring cup. Set aside the fruit.
Take note of how much liquid there is and pour the liquid into a sauce pan or small pot. Add 1 Tbsp. of pectin for every cup. For example, if you end up with 2 cups of liquid, add 2 Tbsp. of pectin.
Stir and cook the liquid over medium heat. Once it starts to simmer, let simmer for 1 minute, then set aside to cool.
Prepare the pie crust. I like to prepare my lattice or vented top crust ahead of time on any non-stick flat surface like plastic wrap, silicone mat, or parchment paper and place on a baking sheet in the fridge until ready to use.
Add the orange zest, salt, and tapioca starch to the strawberry and rhubarb fruit and mix well.
By this point the liquid is relatively cooled down and almost setting. Pour into the fruit mixture and mix.
Pour the mixture into the bottom crust. Dot with the butter and cover with the lattice top.
Brush the top of the pie with milk or cream (optional) and sprinkle with 2 tsp. sugar (also optional, but these optional steps result in a nice looking top.
Place the pie tin on a large baking sheet large enough to catch any drippings. Place in the bottom rack and bake at 425°F for 30 minutes.
Lower the oven temperature to 350°F and bake for an additional 30 minutes. The juices should be bubbling and thick.
Let cool completely! This is a very important step. In fact, sometimes it's better to cool completely and then place in the refrigerator before serving.