Combine 1 cup flour, ⅓ cup oats, ¾ cup sugar and a pinch of salt in a bowl.
Stir in 6 tablespoons melted butter and ½ cup chopped hazelnuts; squeeze into large crumbles and place in the freezer.
Toss 2 pounds chopped rhubarb, ⅓ cup sugar, ¼ cup flour, ½ teaspoon vanilla extract, ½ teaspoon orange zest and ¼ 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.