In a mixing bowl, whisk the flour and sugar together then cut in the cold butter. After you've cut the butter in somewhat, it works well to pinch the mixture between your fingers (to create flakes of butter/flour). It will look like a pile of flour at first, but keep pinching and it will eventually become a crumbly dough.
Press into a 10x15 baking pan and bake for 15 minutes.
Filling & Assembly:
Mix together the filling ingredients in a bowl.
Remove the crust from the oven, sprinkle the rhubarb evenly over it, then pour the filling evenly over the rhubarb.
Return to the oven and bake for 40 minutes or until the filling is lightly browned.
Notes:
Make Ahead: These are best fresh, but you can store them in the fridge too. They can get soggy, so just loosely cover them.