Slice the Tomatoes: Using a sharp serrated bread knife, cut the heirloom tomatoes into thick slabs (about ½-inch thick).
Season the Slices: Lay the tomato slices in a single layer on paper towels or a cutting board. Sprinkle generously with coarse sea salt and freshly ground black pepper on both sides; let sit for 2 minutes.
Prep the Bread: Spread a thick, even layer of mayonnaise edge-to-edge on one side of all 8 bread slices.
Assemble the Sandwich: Divide the seasoned heirloom tomato slabs evenly across 4 of the mayonnaise-coated bread slices, stacking the tomatoes generously. Top each with the remaining bread slices, mayo-side down.
Slice and Serve: Cut each sandwich gently in half with a serrated knife and serve immediately—preferably standing over the kitchen sink.