Pre-heat the oven to 350 and get out a deep 9 x 13 baking dish. Combine the rhubarb, strawberries, sugar and lemon zest in a large bowl. Stir them together. In another small bowl, dissolve the corn starch in the lemon juice, stirring it well. Pour that thick liquid into the big bowl with the rest of the filling and stir it all together thoroughly. Set the bowl aside. Then get out a stand mixer fitted with the paddle attachment. Combine all of the topping ingredients in the bowl and turn the mixer on low. Mix everything together until it is all moist and pebbly from the butter.

Pour the fruit mixture into the pan in an even layer. Then sprinkle the topping evenly on top. Bake the strawberry rhubarb crisp for an hour, until the fruit mixture is really bubbly and thick and the topping is sightly brown. Let it cool for about 10 minutes, then serve it warm with ice cream on top! This also keeps well for a day or two and can be re-heated.