Open-face tarts called galettes are popular French fare. This one, with its tender, flavorful pastry and festive red berry filling, is sure to be a hit among your dessert-lovers. If fresh cherries aren't available, you can use frozen (thawed) ones, which also work beautifully.
To make the dough, combine the flour, sugar, and salt in a mixing bowl. Cut in the butter with a pastry blender.
Blend the ice water and sour cream or yogurt in a measuring cup and sprinkle it over the dry mixture. Stir briskly until the dough gathers into a ball.
Turn the dough out onto a floured surface and, with floured hands, knead it for about 10 seconds, just long enough to smooth it. Flatten the dough into a thick disk, wrap it in plastic wrap, and chill it for at least 2 hours before rolling it out.
When the dough is chilled, line a large baking sheet with parchment paper or lightly greased aluminum foil. Heat the oven to 400 degrees.
Combine the cherries, cranberries, sugar, lemon juice, flour, and vanilla extract in a medium-size mixing bowl. Mix well.
Working on a well-floured sheet of waxed paper, roll the dough into a 12-inch circle; flour the pin and surface of the dough, if needed, to keep the pin from sticking. Invert the pastry over the lined baking sheet and peel off the paper. Pour the filling onto the center of the pastry, spreading it evenly and leaving a 2 1/2-inch border of uncovered pastry all around.
Using a spatula to help lift the dough, fold the edge of the pastry partially over the filling, working your way around the tart. The pastry will form pleats as you go. Using a pastry brush, lightly moisten the exposed pastry with water and then sprinkle it with sugar. Cut the tablespoon of butter into small pieces and scatter it over the fruit.
Bake the tart on the center oven rack for about 40 minutes, until the fruit is bubbly and the pastry is a rich golden brown. Cool the tart on the pan for 5 minutes, then slide the tart and the paper or foil lining onto a wire rack. Cut it into wedges and serve warm or at room temperature. Makes 8 servings.