1To make the pastry, combine the flour, sugar and salt in a bowl or in the bowl of a food processor; stir or pulse to blend. Add the butter and/or lard, cut into pieces, and blend with a pastry cutter, fork or your fingers, or pulse in the food processor until the mixture is almost combined, with lumps of fat the size of a pea remaining.
2If you used a food processor, pour it into a bowl and add the water. Stir until the dough comes together. Shape into a disc and let it rest on the countertop for 20 minutes, or wrap it in plastic and chill for up to 3 days.
3When you're ready to bake, preheat the oven to 400F. On a lightly floured surface, roll the pastry out to about a 10-inch circle. Transfer to a pie plate. Put the rhubarb and raspberries into a large bowl, and in a small bowl, stir together the sugar and cornstarch. Sprinkle over the rhubarb and berries and stir to coat.
4Dump the fruit - along with any sugar that accumulated in the bottom of the bowl - into the pastry. Fold over the edges wherever they want to fold. If you like, make the crumble by blending together all the ingredients in a bowl or in the bowl of a food processor. Sprinkle over the fruit, squeezing as you go to create larger clumps.
5Bake for 1 hour, or until bubbly and golden. If the topping or crust is browning too quickly, cover it with some foil or reduce the temperature to 350F.
6Serve warm, with vanilla ice cream.