How to Make It
In a food processor, pulse the flour and salt. Add the chilled butter and pulse until it's the size of small peas. Add the frozen butter and pulse until it's the size of small peas. Drizzle the vinegar mixture over the flour mixture. Pulse just until the dough starts to come together. Turn out onto a lightly floured surface and knead gently to form a smooth dough. Divide the dough in half and flatten into 2 disks. Wrap in plastic and refrigerate until firm, at least 2 hours.
On a lightly floured surface, roll out one disk of dough to a 12-inch round, 1/8 inch thick. Transfer to a 10-inch metal pie plate and freeze for 20 minutes. Roll out the remaining dough 1/8 inch thick. Transfer to a baking sheet and freeze for 20 minutes.
In a small bowl, whisk 1 cup of sugar with the tapioca and salt. In another bowl, toss the cherries with the raspberries and blueberries. Sprinkle with the sugar mixture and toss gently.
Mix the flour and the remaining 1 teaspoon of sugar. Sprinkle in the crust; tap out the excess. Spoon the filling into the crust.
Using a fluted pastry wheel, cut the remaining dough into 3/4-inch-wide strips. Arrange the strips across the pie in a lattice pattern. Trim the overhanging strips to 1/2 inch and press them to the crust. Pinch the rim between your fingers to flute it. Freeze the pie for 1 hour.
Preheat the oven to 400°. Brush the cream over the lattice crust and rim and sprinkle with sugar. Bake in the upper third of the oven for 1 hour, or until the crust is richly browned and the filling starts to bubble. If necessary, cover the pie with foil for the last 10 minutes of baking. Let cool on a rack until warm before serving.