In separate bowls, toss the sliced rhubarb with 3 tablespoons of sugar and the hulled strawberries with 3 tablespoons of sugar. Let them sit for 20 minutes to release their natural juices—this maceration step draws out moisture and concentrates the fruit flavor. After 20 minutes, strain the fruit through a fine-mesh sieve into a bowl, reserving the juices (you should have about 1/2 cup liquid). Set the drained fruit aside; I discard the excess juice since the cornstarch will provide enough thickening power without making the filling overly wet.

Preheat your oven to 350°F. While the oven heats, place the chopped pecans in a small dry skillet over medium heat and toast for 8 minutes, stirring occasionally, until fragrant and lightly golden. Transfer to a plate and let cool slightly, then finely chop into 1/4-inch pieces if not already done. In a large bowl, whisk together the gluten-free oats, gluten-free oat flour, brown sugar, salt, and cinnamon. Set this dry mixture aside—toasting the pecans separately adds a deeper, more complex nutty flavor that's much better than raw pecans.

In a medium bowl, combine the drained fruit from Step 1 with the lemon zest, lemon juice, vanilla extract, and cornstarch. Stir gently but thoroughly until the cornstarch is evenly distributed throughout the fruit and there are no lumps—this ensures the filling will thicken uniformly as it bakes. The cornstarch will absorb any remaining moisture from the fruit and set up during baking to create a cohesive but not overly thick filling.

Melt the 1/2 cup unsalted butter in a small saucepan over low heat or in the microwave. Pour the melted butter into the bowl with the dry crumble mixture from Step 2 and stir until the oat mixture is evenly moistened and clumpy in texture—you want some texture, not a wet paste. Fold in the toasted pecans from Step 2, distributing them evenly throughout the topping.

Lightly grease a 9x13-inch baking dish (or similar size) with the 2 teaspoons of butter. Spread the fruit filling from Step 3 evenly across the bottom of the prepared pan. Distribute the crumble topping from Step 4 evenly over the fruit, breaking up any large clumps and leaving it slightly rustic rather than compacted—this allows for better browning and crispier texture. Bake at 350°F for 25–30 minutes until the filling is bubbling at the edges and the topping is golden brown.

Remove the crumble from the oven and let it cool for at least 10 minutes before serving. This resting period allows the filling to set slightly and makes portioning easier. Serve warm or at room temperature with a scoop of vanilla ice cream if desired—the cold ice cream creates a beautiful temperature and texture contrast against the warm, jammy filling and crispy topping.