Preheat oven to 350º F. Line a 12x17 jelly roll pan with parchment paper so you can easily lift the bars out once they've cooled.*
To make the crust:
Cream the butter and sugar together with a mixer until light and fluffy, then add in vanilla. Add flour until just incorporated. Dump dough in baking pan and press with your hands until it evenly covers the bottom of the dish. Bake for 20 minutes until slightly golden brown.
To make the filling:
In a small sauce pan, heat strawberries, rhubarb, lemon juice and zest, and sugar over medium heat until fruit is soft, about 15min. Allow mixture to cool, then blend until smooth in blender or food processor.
Add strawberry rhubarb mixture to a medium sized bowl and add flour, eggs and salt. Stir to combine. Pour the mixture into the crust. Mixture will be very soft and wet but will thicken as it cooks. Bake for 25 minutes depending on your oven and the sized baking dish you use. Do not allow to brown.
Let cool to room temperature, slice into small squares, and store in sealed container in refrigerator.
Dust with powdered sugar prior to serving.
Notes
* If you prefer more filling to crust ratio, cut the ingredients for the crust in half and use an 8x8 or a 9x9 baking pan. Cooking times will need to be increased to accommodate the thicker filling.