Evie's Rhubarb Pie with Oatmeal Crumble

Toss the rhubarb with 1 1/2 cups of sugar and the tapioca in a large bowl. Drizzle with the melted butter and toss again to coat. Pour the rhubarb into the pie crust, and set aside. Mix together the oats, 1/4 cup sugar, flour, softened butter, ...