Place 2 slices white sandwich bread on a cutting board. Spread 2 tablespoons mayonnaise evenly over the bread. Use a serrated knife to cut 2 (1/2-inch-thick) slices from 1 large tomato. Sprinkle the slices lightly with kosher salt and a few grinds of freshly ground black pepper. Stack the tomato slices on one slice of the bread. Close the sandwich with the other slice of bread and use a serrated knife to cut in half. This sandwich is best eaten immediately.