FILLING – 2 heaping cups thinly sliced rhubarb Note: if your rhubarb is thick, slice the whole stalk lengthwise and then chop – 1/4 cup granulated sugar – 1 tsp vanilla bean paste – 1 Tbsp Instant Clearjel thickener, or substitute cornstarch CRUST AND CRUMBLE TOPPING – 1 cup unsalted butter (two sticks) at room temperature – 2/3 cup granulated sugar – 2 tsp vanilla bean paste, you can also use extract, or the seeds of a vanilla bean – 1/4 tsp salt – 2 cups all purpose flour – 3 Tbsp rolled oats, for the crumble topping