Preheat the oven to 400 degrees F (200 degrees C). Line pie crust with a double layer of aluminum foil and a layer of pie weights or dried beans.

Mix together flour and brown sugar in a bowl. Add cold butter cubes and cut in with your fingers until mostly combined with flour mixture, allowing some medium-sized chunks of butter to remain. Place crumble mixture in the freezer.

Bake pie crust in the preheated oven until edge of crust is slightly browned, about 8 minutes. Remove from the oven, carefully remove the foil and weights, and set aside. Leave oven on.

Combine rhubarb, strawberries, white sugar, cornstarch, lemon juice, thyme, and rosemary in a large bowl; mix until fruit is well coated, and transfer mixture into the pie crust. Remove crumble mixture from the freezer and sprinkle over top.