Step 1

Preheat the oven to 375°F (190°C). Put a 2 qt baking dish on a baking sheet, and set aside.

Step 2

In a large bowl, combine the sugar, cornstarch, cinnamon, and lemon zest. Add the chopped rhubarb and toss until evenly coated. Add the lemon juice and toss it again. Pour it into the baking dish and set it aside.

Step 3

In a separate bowl, combine the gluten free flour, oats, granulated sugar, brown sugar, salt, and cinnamon. Add the butter and use your hands to pinch the butter and incorporate it into the flour mixture until it resembles course crumbs.

Step 4

Evenly sprinkle it over the rhubarb mixture and bake it for 45 minutes. Let it cool for at least 20 minutes before serving.