Preheat the oven to 400 degrees.
Rinse the strawberries in a colander and let them drain.
Slice the strawberries in half and add to an 8 or 9-inch square baking dish.
If you are using fresh rhubarb, rinse and slice the rhubarb stalks into ½-inch chunks. If you are using frozen rhubarb, the fruit should already be in chunks.
Add the rhubarb to the strawberries in the baking dish and mix with flour and sugar.
Chop the butter into chunks in a medium-sized microwave-safe mixing bowl, then melt the butter in the microwave for about 45 seconds.
After the butter is melted, add the brown sugar, flour, oatmeal, and cinnamon to the bowl and mix.
When the topping mixture is combined, distribute evenly over the top of the strawberries and rhubarb in the baking dish.
Bake for 40 minutes. The rhubarb and strawberry filling should be bubbly.
Serve warm with vanilla ice cream.