Combine 1 cup flour, 1/3 cup oats, 3/4 cup sugar and a pinch of salt in a bowl. Stir in 6 tablespoons melted butter and 1/2 cup chopped hazelnuts; squeeze into large crumbles and place in the freezer. Toss 2 pounds chopped rhubarb, 1/3 cup sugar, 1/4 cup flour, 1/2 teaspoon vanilla extract, 1/2 teaspoon orange zest and 1/4 teaspoon salt in a 8-by-8-inch glass or ceramic baking dish. Scatter the crumble on top and bake in a preheated 375 degrees F oven until golden and bubbly, 45 minutes. Let cool for 15 minutes; serve with whipped cream.