Preheat your oven to 375°F and grease a 9x13 inch baking dish (or similar size) with butter or cooking spray. While the oven heats, slice the rhubarb into 1-inch pieces and measure out all your ingredients. I like to cut the cold butter into 1/2-inch cubes and keep it in the freezer until I'm ready to use it—this helps create the best crumbly texture in the topping.

In a medium bowl, combine the sliced rhubarb with 1 1/4 cups sugar, 1 large egg (room temperature), 2 tablespoons gluten free flour, and 1/2 teaspoon cinnamon. Stir gently until the rhubarb is evenly coated and the egg is fully incorporated. The egg and flour will help thicken the filling as it bakes, creating a nice jammy consistency.

In a separate bowl, whisk together 1 3/4 cups gluten free flour, 1 cup firmly packed brown sugar, 1 cup oats, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Add the cold butter cubes and use a pastry cutter, fork, or your fingertips to work the butter into the dry mixture until it resembles coarse breadcrumbs with pea-sized pieces of butter still visible. I prefer using my fingertips for better control—just work quickly so the butter stays cold and creates those lovely crumbly pockets.

Pour the rhubarb filling from Step 2 into the prepared baking dish, spreading it into an even layer. Scatter the crumble topping from Step 3 evenly over the rhubarb, breaking up any large clumps with your fingers but leaving some texture. Bake for 35 to 40 minutes until the topping is golden brown and the filling is bubbling around the edges. The fruit should be tender when pierced with a fork.

Remove the crumble from the oven and let it cool for at least 10 minutes before serving—this allows the filling to set slightly and makes it easier to scoop. Serve warm with vanilla ice cream or whipped cream, or enjoy it cold the next day (the flavors actually deepen and the texture improves as it sits).