In a large bowl, mix white sugar, 3 tablespoons flour, strawberries, and rhubarb. Place the mixture in a lightly-buttered 9x13 inch baking dish.
Mix 1 ½ cups of flour, brown sugar, butter, and oats until crumbly (mixing with your hands is the easiest, though you could also use a pastry blender). Crumble on top of the rhubarb and strawberry mixture.
Bake for 45 minutes, or until crisp and lightly browned. Serve with vanilla ice cream if you’d like.