Whisk flour, baking powder and 1/2 teaspoon salt; cut in butter until mixture resembles coarse crumbs. Stir in sour cream until dough forms a ball. Shape into a disk; cover and refrigerate until firm enough to roll, about 2 hours., Meanwhile, place tomatoes in a colander; toss with remaining salt. Let stand 15 minutes., Preheat oven to 425°. On a floured sheet of parchment, roll dough into a 12-in. circle. Transfer to a baking sheet., Place cheese slices over crust to within 2 in. of edge; arrange tomatoes over cheese. Fold crust edges over filling, pleating as you go and leaving center uncovered. Bake until crust is golden brown and cheese is bubbly, about 25 minutes. Cool 10 minutes before slicing.