Toast all slices of sourdough bread until lightly crisped and set them aside. Cut the tomatoes into thick rounds, each slightly over ¼ inch in thickness, and sprinkle with flaky sea salt or kosher salt; allow them to sit briefly.

Slice the halloumi cheese into strips approximately ¼ inch thick, ensuring they are no longer than the bread slices. Pat the slices gently with a clean tea towel or paper towel to remove surface moisture, minimizing splatter during frying.

Heat the olive oil in a medium skillet over medium heat until it reaches a temperature that causes the cheese to bubble slightly upon contact. Carefully lay the halloumi strips in the pan using kitchen tongs, spacing them apart to prevent sticking. Cover the pan to reduce splattering.

Cook the halloumi until the underside is deeply golden, about 2 to 4 minutes. Adjust heat if necessary—raise if browning is slow, lower if it browns too quickly or smokes. Flip each slice carefully with tongs, re-cover, and cook the other side until equally golden, another 2 to 4 minutes.

Transfer the cooked cheese to a paper towel-lined plate to absorb excess oil. Sprinkle generously with freshly ground black pepper and let cool slightly.

To assemble, generously spread mayonnaise on half the toasted bread slices. On the remaining slices, layer with lettuce leaves, followed by tomato slices arranged to fully cover the surface. Add a layer of halloumi slices, overlapping as needed. Top with the mayo-coated slices of bread, facing down.

Serve the sandwiches whole, or cut them in half using a sharp chef’s knife. For optimal texture and flavor, enjoy fresh. Refrigerate leftovers in an airtight container for up to 24 hours.