Preheat oven to 350 degrees.

In a large pot, mix the rhubarb, white sugar, cornstarch, vanilla and strawberry jam together.  Cook over medium heat until the mixture becomes thick and "jammy".  This process will vary in time depending on whether you are using fresh or frozen rhubarb (fresh takes longer to break down) - you want to make sure the rhubarb is beginning to get soft. Remember, it will thicken a bit as it cools.

In a large mixing bowl, combine the flour, oats, salt, baking powder, baking soda and brown sugar.  With your hands (always easiest that way), mix in the butter.  Rub the butter between your fingers to incorporate all of the dry ingredients with the butter to get a crumbly mixture (see picture below)

Line a 9x13" baking pan with parchment and place half of the oat/flour/butter mixture into the pan; press down firmly to create a good crust.

Pour the rhubarb mixture over the base.

Sprinkle the remaining crumb mixture over the rhubarb (don't press down this time) and bake in an oven until golden (about 40 minutes).

Remove from oven and let cool (if you can wait!).  Serve warm with good quality vanilla ice cream!