In a 9 x 13 inch pan, stir together strawberries, rhubarb, lemon juice, sugar, cornstarch and salt until well combined. Let sit for 10 to 15 minutes to allow the cornstarch to fully dissolve.
Crisp Topping
In a large bowl, mix together the flour, oats, baking powder, salt, sugars, and lemon zest. Add the melted butter and stir until the mixture has coarse clumps. Chill in the refrigerator for 10 minutes.
Crumble the topping in your hands evenly over top of the fruit mixture.
Bake for 45 minutes until the fruit mixture is bubbling and the topping is golden brown. You may want to place the pan on top of a cookie sheet lined with foil to catch any drippings.
Remove from the oven and let cool for 15 minutes before serving. Serve warm with a scoop of vanilla ice cream.