Wash the rhubarb thoroughly under cold water, chop into small pieces, and combine with sugar, cornstarch, vanilla extract, and lemon juice. Let sit for 10-15 minutes.
Mix together the gluten-free rolled oats, almond flour, brown sugar, cinnamon, and salt. Pour in the melted butter and stir until crumbly.
Press half of the oat mixture into the bottom of a greased 9x9-inch baking dish. Spread the rhubarb mixture evenly over the crust and sprinkle the remaining oat mixture on top.
Preheat oven to 350°F. Bake for 30-35 minutes until golden brown and bubbling. Allow to cool completely before cutting into squares.