Mix together flour, oatmeal, brown sugar and cinnamon; stir in butter. Press half of mixture into greased 2 quart casserole dish.
Mix rhubarb, strawberries and sugar in a large saucepan; let set for 2 minutes. Stir in cornstarch and vanilla. Cook and stir until thickened. Pour over crust; top with remaining crumb mixture.
Bake for 30 minutes at 350 degrees Fahrenheit until top is golden brown, fruit is bubbling and cooked through..
Video
Notes
Frozen rhubarb can be used instead of fresh rhubarb, but you will need to add about 1/2 - 3/4 Tablespoons to the sauce mixture to get it thicken due to the extra moisture content in the rhubarb.