Preheat your oven to 375 degrees F. This step is crucial to ensure even baking of the crisp.
In a medium-sized bowl, prepare the oat crumble topping. Combine the oats, flour, brown sugar, baking powder, and cinnamon with a fork or a pastry blender until it resembles small crumbs. Refrigerate the mixture while you prepare the filling.
Next, slice the rhubarb stalks in half lengthwise and then cut them into 1/2-inch pieces, aiming for about 3 cups of chopped rhubarb. You want it to be fresh and vibrant.
In a large bowl, mix together the chopped rhubarb, strawberries, sugar, vanilla, orange juice, orange zest, and cornstarch. Stir until everything is well combined.
Pour the fruit mixture into an 8 or 9-inch square baking dish. If you're using a shallow baking dish, you might want to place it on top of a foil-lined baking sheet to catch any potential spills.
Now, sprinkle the crumble topping evenly over the fruit mixture. Make sure to cover it adequately to create that delicious, crunchy layer.
Place the baking dish in the preheated oven and bake for about 35 minutes. You'll know it's done when the topping is golden brown and the fruit is bubbling up around the edges.
Once baked, remove it from the oven and allow it to cool for about 10 minutes before serving. This waiting time helps the filling set a bit.
Serve warm with a scoop of vanilla ice cream or whipped cream for an extra indulgent touch!
