Core and slice the tomatoes crosswise 0.25–0.5 inch thick.
Arrange the slices on a rack or paper towels; sprinkle evenly with the kosher salt and let stand 10–15 minutes, until moisture beads on the surface, then gently pat dry.
Spread the mayonnaise over one side of each slice of white sandwich bread, edge to edge.
Lay half of the slices on two mayo-coated slices in an even, slightly overlapping layer.
Season evenly with the black pepper.
Top with the remaining slices, mayo side down, and press gently so the juices mingle.