In a large bowl, combine rhubarb, strawberries, brown sugar, granulated sugar, cornstarch, salt, and lemon juice. Let sit for 10 minutes.
In a separate, smaller bowl, prepare the crumb topping by whisking together flour, brown sugar,and a pinch of salt. Add melted butter and mix until crumbly.
Place a pie crust in the bottom of a pie pan and flute the edges.Drain excess liquid from the fruit mixture. Pour the fruit into the pie crust. Top with crumb topping, gently pressing it down.
Place the pie on a foil-lined baking sheet and bake at 400°F for 20 minutes. Reduce oven temperature to 350°F and continue baking for 25–30 minutes. Cover loosely with foil if topping darkens too much.
Let the pie cool for at least 3 hours on a wire rack before slicing and serving!