Preheat oven to 375˚F.
In a medium bowl, combine the flour, baking powder, 6 tablespoons of the sugar, and lemon zest. Stir in the melted butter, mixing until clumps form. Cover and place in the refrigerator.
In a 9-inch pie dish, combine the rhubarb, strawberries, and lemon juice. Sprinkle with cornstarch and toss to incorporate.
Remove topping from refrigerator. If needed, break up the topping so that it forms crumbles again. Cover the fruit evenly with the crumble topping, creating a thick layer. Place pie dish on a parchment-, foil-, or Silpat-lined baking sheet, and place on middle rack of oven. Bake 40-50 minutes, until the crumble is beginning to turn golden and the fruit is bubbling. Allow to cool at least 20 minutes on a wire rack before serving.