Preheat the oven to 356°F (180°C) and line a 9x13 inch baking dish with parchment paper.

In a mixing bowl, combine flour, oats, brown sugar, baking powder, and salt. Whisk until uniform.

Cut cold butter into cubes and add to the dry mixture. Mix until it resembles coarse breadcrumbs.

Press two-thirds of the crumble mixture into the bottom of the prepared baking dish.

Pre-bake the crust for about 10 minutes until slightly golden.

In a saucepan, combine rhubarb, blueberries, sugar, cornstarch, cinnamon, and nutmeg. Cook over medium heat for about 15 minutes.

Stir in lemon zest and juice before removing from heat and allowing to cool slightly.

Spoon the fruit mixture evenly over the warm crust, then sprinkle the reserved crumble mixture on top.

Bake for an additional 25-30 minutes until the topping is golden brown and filling is bubbling.

Allow to cool in the pan for at least 2 hours before cutting into squares.