• Put the oats, flour, baking soda, salt, and brown sugar in a food processor and pulse a few ties to blend
• Add the butter and about half of the nuts and process until the mixture starts to clump together
• Put half of the mixture nto a 9x13-inch baking dish.
• Add the remaining nuts to the rest of the oatmeal mixture, pulse onece just to blend, and reserve in the refrigerator for the crumb topping
• Press the crust mixture into the baking dish to form an even layer over the bottom. Chill while you make the rhubarb filling

Make the filling

• Heat the oven to 375 degrees
• Put the chopped rhubarb, sugar, cornstarch, vanilla, and ¼ cup of water into a medium saucepan over medioum-high hea, bring to a boil, stirring constantly, and then reduce the heat to a simmer
• Continue cooking and stirring until the filling is thick and jam-like, 15 to 25 minues. Let the filling cool and then pour it over the bottom crust.
• Crumble the remaining topping mixture over the top as evenly as possible; it's okay if the rhubarb peeks through
• Bake until the bottom crust and topping are nicely golden and the filling is bubbling just a bit around the edges, 25-35 minutes.
• Cool completely before cutting into approximately 2-inch squares