Recipe adapted from Bon Appétit June 1989
Preheat oven temperature to 350°F. Combine oats, flour, sugar, and cinnamon in a bowl. Add butter and squish with hands until butter is fully incorporated and mixture is uniformly crumbly. Transfer mixture to medium bowl.
Mix rhubarb, strawberries, sugar, cornstarch, and lemon juice in heavy large saucepan. Let stand 30 minutes. Bring to boil over medium heat, stirring constantly. Reduce heat and simmer until juices thicken, about 5 minutes. Strain fruit with a strainer and save the syrup for a cocktail or two.
Pour filling into baked crust. Cover with topping. Bake about 20 minutes--check on crust, and cover with foil if browning too quickly. Cool on rack. Serve warm or at room temperature.