Preheat your oven to 350 degrees F. Rub about a half of a tablespoon of butter all over the inside of a 11-inch x 9-inch baking dish. In a small bowl, add the vanilla bean and sugar; rub the vanilla bean throughout the sugar until it’s evenly distributed. In the baking dish (no need to dirty a bowl for this), combine vanilla sugar, the sliced rhubarb, strawberries, lime juice and salt. Set aside while you make the crisp topping.

In a small to medium bowl (feel free to use the one you used to mixed the vanilla sugar), add the flour. Using a box grater, grate the butter atop the flour and mix throughout until the butter resembles small peas. Mix in the brown sugar, rolled oats and salt. Pour the crisp topping over the rhubarb filling and transfer to the oven to bake for 30 minutes, until bubbly. Combine the egg yolk and milk in a small bowl and brush the top of the crisp with the egg wash mixture. Return it to the oven and bake for an additional 15 minutes, until the top is golden brown and has hardened slightly and the rhubarb and strawberries are all juicy and wonderful. Serve this crisp warm with vanilla ice cream.