Preheat your oven to 425°F (220°C).
Prepare the fruit filling: In a large mixing bowl, combine the strawberries and rhubarb.
Add sugar, flour, lemon juice, and cinnamon to the fruit mixture; stir gently to combine.
Let the filling sit for 15 minutes to allow the juices to meld.
Roll out the pie crust and place it in a pie dish, trimming the edges as needed.
Pour the fruit filling into the prepared pie crust and dot with butter pieces.
Cover the pie with a second crust or lattice, sealing the edges well.
Cut slits into the top crust for steam to escape and sprinkle sugar on top if desired.
Bake in the preheated oven for 15 minutes, then reduce heat to 350°F (175°C) and bake for an additional 35-40 minutes.
Allow the pie to cool for at least 2 hours before slicing.