Preheat oven to 350 degrees. Prepare a square 9x9" baking dish by lining it with parchment paper so that the bars will be easy to remove and cut once they have cooked and cooled. Or just spray it with cooking spray if you don't have parchment paper on hand.

In a mixing bowl, combine the flour, oats, soda, salt, sugars and walnuts. In a separate bowl, stir the vanilla into the melted butter, then add to the butter and vanilla to the oat mixture and stir to incorporate. When the crumb mixture is evenly moist, press ⅔ of it into the bottom of your prepared baking dish. Set aside the remaining ⅓ of the crumb mixture for the top layer.

In a separate mixing bowl, combine the chopped strawberries and rhubarb. Toss with the lemon juice, then add the sugar and cornstarch and stir to coat the fruit. Pour the strawberry rhubarb mixture evenly over the the bottom crumb layer, then sprinkle with the remaining ⅓ crumb mixture. Bake in the preheated oven for 45-50 minutes, until the top is golden brown and the filling is bubbling.

Remove from oven and allow to cool completely before cutting into bars.