• Preheat the oven to 350 degrees with the rack in the middle position.
• Remove and discard the rhubarb leaves and wash the rhubarb stalks removing any dirt.  On a cutting board and sharp knife, cut the stalks into 1/2-inch pieces and place them in a large bowl. Wash the blueberries, pat dry, and add to the bowl.  Combine rhubarb and blueberries with brown sugar and lemon juice. Set aside and allow the fruit to macerate while you mix the oat crisp topping.
• In a separate medium bowl cream the butter and brown sugar until smooth. Add the flour and old-fashioned oats, mixing until all of the dry ingredients are incorporated.
• After the fruit has been allowed to rest for a few minutes, you'll notice some liquid in the bowl from the fruit. Sprinkle flour over the fruit mixture and toss until all of the flour is absorbed into the juice and is coating the fruit. 
• Pour the fruit mixture into a 9 x 13 baking dish or large cast iron skillet. Crumble the oat crisp topping evenly over the top of the fruit.
• Bake for 35-40 minutes or until the top of the crisp is starting to turn light golden brown. Move to a wire rack to cool.

Storing

• Blueberry rhubarb crisp is best the day it's baked. Store any leftovers in an airtight container at room temperature for 2 days or in the refrigerator for up to 4 days. The crisp topping will start to soften once it's stored.