Preheat oven to 425°F. In a large bowl, combine the rhubarb, strawberries, lemon juice, sugar, cornstarch, and cinnamon. Stir to coat and then pour into a greased 2 quart (8-9" size) baking dish. Bake for 10 minutes.
While the filling is in the oven, whisk together the flour, brown sugar, baking powder, and salt. Toss the grated butter into the flour mixture. Stir in the boiling water, just until combined, leaving plenty of little lumps of butter.
Remove the filling from the oven and spoon the lumpy batter on top of it. (I like to use my smallest cookie scoop to do this.) Mix together the cinnamon and sugar and sprinkle it onto the cobbler topping.
Bake until the crust is golden and a toothpick inserted into the crust comes out clean, about 20-24 minutes. Let cool for 10 minutes. Serve warm with a scoop of vanilla ice cream, if desired.