In a large bowl, combine the flour, sugar, and salt. Using two forks (or a food processor) cut in the butter until the mixture resembles coarse cornmeal. Add the buttermilk and vinegar and stir with a wooden spoon until just combined.
Transfer to a lightly floured surface and work into a large flat disc. Wrap tightly in plastic wrap and chill for a half hour or until ready to use.
In a small bowl, combine the strawberries and tomatoes. Sprinkle the sugar and salt over the bowl and gently toss to combine.
Set oven to 350 degrees. Line a baking sheet or two with parchment paper.
Whisk together the egg and cream to create the egg wash. Set aside
Transfer chilled dough to a lightly floured surface. Roll out dough to about 1/4-inch or less thickness. Using a biscuit cutter or the open end of a similarly sized glass, cut the dough into rounds. Roll each round again – once in each direction to create a slightly thinner and larger circle. Spread the rounds onto the lined baking sheets.
Carefully drain any excess liquid that has pooled at the bottom of the strawberry-tomato mixture. Onto each round, spoon a tablespoon or so of filling. Using your fingers, gently fold over and slightly crimp the edges of each dough round to create a little ring around the filling – the middle will be left exposed. Lightly brush with egg wash and sprinkle with sugar. Repeat with remaining dough rounds. Place the baking sheets in the oven and bake – rotating halfway through – till they begin to turn a golden brown and the crust has set, about 20 minutes. Transfer to a wire rack to cool. Best eaten on the porch with iced tea.