Preheat the oven to 350 degrees. For the fruit, toss the rhubarb, strawberries, 3/4 cup of the granulated sugar and the orange zest together in a large bowl. In a measuring cup, dissolve the cornstarch in the orange juice and then mix it into the fruit. Pour the mixture into an 8-by-11-inch baking dish and place it on a sheet pan lined with parchment paper. (That’s to catch any spills from the dish.) For the topping, in the bowl of an electric mixer fitted with the paddle attachment, combine the flour, the remaining 1/2 cup granulated sugar, the brown sugar, salt and oatmeal. With the mixer on low speed, add the butter and mix until the dry ingredients are moist and the mixture is in crumbles. Sprinkle the topping over the fruit, covering it completely, and bake for 1 hour, until the fruit is bubbling and the topping is golden brown.