Preheat oven to 375. Grease a 9x13 baking pan.
In one bowl, mix together the rhubarb, strawberries, sugar, cornstarch, and lemon until coated. Pour into the pan and spread evenly.
In another bowl, stir together flour, oats, brown sugar, salt, nutmeg, and zest. Using a pastry blender, cut in the butter until crumbly. Sprinkle crumble evenly over strawberry-rhubarb layer.
Bake for 40-45 minutes until bubbly and golden. Serve hot with vanilla ice cream.