First - gather all of your ingredients and get out a big pot.
In a large pot mix together the first 5 ingredients and bring to a boil.
Then, lower the temperature to medium and let simmer for about 10 minutes. Stir often.
While the rhubarb is cooking grab your pastry cutter and mix the rest of the ingredients together to create a crumble.
Once the rhubarb is done, pour it out on a cookie sheet (that has a lip) and place it in the freezer to cool quickly - only about 10-15 minutes, while you keep working on the crumble.
Now - time to layer! Put down 2/3 of the crumble and pack it in the 9x13 pan. Gently pour over the rhubarb mixture. Then top with the remaining 1/3 of the crumble.
Bake in the oven for 30-35 minutes at 375 degrees.
Take it out once it's golden brown and then ensure you let it cool before you cut it.