Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x9 inch square baking dish or a 2-quart oval/round baking dish.
- Prepare the Fruit Filling: In a large bowl, gently combine the chopped rhubarb, blueberries, 1/2 cup granulated sugar, 2 tablespoons of flour (or cornstarch), lemon zest, and vanilla extract. Toss until evenly coated, then pour the mixture evenly into the prepared baking dish.
- Make the Topping (Dry Mix): In a separate medium bowl, whisk together the 1 cup flour, rolled oats, both sugars (light brown and 1/4 cup granulated), cinnamon, and salt.
- Cut in the Butter: Add the chilled, cubed butter to the dry ingredients. Using your fingertips or a pastry blender, cut the butter into the dry mixture until the texture resembles coarse breadcrumbs with some pea-sized lumps remaining. Do not overwork the mixture.
- Assemble: Sprinkle the entire crisp topping evenly over the fruit filling in the baking dish, ensuring the fruit is mostly covered.
- Bake: Place the dish on a central oven rack. Bake for 40–45 minutes, or until the topping is deep golden brown and the fruit filling is visibly bubbling thickly around the edges.
- Rest & Serve: Allow the crisp to cool on a wire rack for at least 15–20 minutes before serving. Serve warm, ideally with vanilla bean ice cream or heavy cream.