Preheat your oven to 375°F
Pre-bake your pie crust according to recipe instructions or package directions.
Remove leaves and root portion of rhubarb stalk. Discard.
Slice rhubarb stalks into 1/2 inch to 1 inch slices and put into a large mixing bowl.
In a small bowl, mix the sugar and flour.
Add the melted butter to the cut rhubarb and stir well.
Add the sugar mixtture to the rhubarb and stir until all the rhubarb is thoroughly coated.
Pour the rhubarb mixture into the pre-baked crust. The mixture should be heaping. When the pie bakes, it will reduce in size and be a full flat-topped pie.
Make the crumble topping, In a small bowl, mix the brown sugar, flour, oatmeal salt and canola oil.
Stir the crumble topping mixture until wet and sticky.
Place the pie onto a baking sheet covered in aluminum foil.
Place topping onto the pie, patting down as you go, until the entire top is covered. Scoop any mixture that falls off and pat onto the top.
Bake on center rack of the oven for approximately 50 minutes ( bake on a foil-lined baking sheet - this prevents any juices that boil over from landing in your oven)
Let cool for a 1/2 hour (at least) before serving. (this isn't necessary, but it allows the juices to settle before cutting)
Serve warm with a scoop or two of ice cream.