Preheat oven to 400 degrees F (200 degrees C). Whisk 1 cup of flour and salt in a bowl.
Cut 1/3 cup of butter into the flour mixture with a pastry cutter until the mixture resembles coarse crumbs. Moisten with water, 1 tablespoon at a time, just until the mixture holds together. Shape the dough into a ball, and roll out into a 12-inch circle on a floured work surface. Crust will be thin. Fold the dough gently into quarters, and lay into a 9-inch pie dish; unfold the dough and center the crust in the pie dish. Trim the crust to 1/2 inch of overhang, and crimp or flute the edge of the crust. Refrigerate crust while making filling.
Mix 1 1/4 cups of sugar, 1/3 cup of flour, cinnamon, and nutmeg in a bowl until thoroughly combined. Mix in the rhubarb and strawberries, and pour into the crust-lined pie dish. Sprinkle with pecans. Mix 1 cup of flour with 2/3 cup of sugar in a bowl, and cut 1/3 cup of butter into the mixture with a pastry cutter until the mixture resembles coarse crumbs; sprinkle the crumb topping evenly over the pie filling. Cover the edge of the pie with strips of aluminum foil.
Bake in the preheated oven until the crumb topping is golden brown and the filling bubbles around the edges, 50 to 60 minutes. Remove foil for the last 10 minutes of baking to brown the pie edge.