In a large bowl, toss together the rhubarb, sugar, cornstarch, and salt. Spread evenly in the bottom of an 8" square baking pan.
In a small saucepan, melt the butter over low heat. Remove from heat and set aside to cool a few minutes. Stir in the ½ cup flour blend, oats, brown sugar, and cinnamon. Spread mixture evenly over top of rhubarb mixture.
Bake for 40 to 45 minutes, or until browned and bubbly.
Serve warm with a little lactose-free vanilla ice cream or fresh whipped cream!