1. For the crumb layer: Preheat oven to 350'F.  Butter a 9x9 inch baking pan.

2. In a mixing bowl, whisk together flour and baking soda.  Add in oats, brown sugar, and granulated sugar and whisk to blend.  Use fingertips to break up small clumps of brown sugar.

3. Whisk vanilla into the melted butter, then pour melted butter over oat mixture.  Stir with a spatula until mixture is evenly moistened.

4. Press 2/3 of the mixture into the prepared baking dish, set aside remaining 1/3.

5. For the filling: Add diced strawberries and rhubarb to a medium mixing bowl and toss with lemon juice.

6. In a small mixing bowl, whisk together sugar and cornstarch.  Pour sugar mixture over strawberry mixture and toss to coat.

7. Pour strawberry mixture over bottom crumb layer in a baking dish.  Evenly sprinkle remaining 1/3 crumb mixture over top.

8. Bake in a preheated oven until top is golden brown and crisp and filling is bubbling, about 45-50 minutes. 

9. Cool on a wire rack until warm and then cut into 12 bars.