Step 1 Lightly toast each slice of bread, and set aside. Slice your tomatoes into thick rounds (a little over ¼″ thick) and sprinkle them lightly with flaky salt or kosher salt. Set aside.
Step 2 Slice the halloumi into ¼” wide strips no longer than your bread. Using a clean tea towel or paper towel, gently pat dry any pools of moisture on the cheese (less water means less splattering). Place the cheese near the stove.
Step 3 In a medium skillet (I love to use cast iron) over medium heat, warm the olive oil over medium heat. We want the oil warm enough that a slice of cheese, once added, quickly starts bubbling around the edges. Using kitchen tongs, carefully place slices of halloumi into the hot oil (be careful, the oil can splatter). Leave a little room around each piece of cheese so they don’t stick together (I usually cook the cheese in two batches). I recommend covering the skillet to contain any splatters.
Step 4 Cook the cheese until the undersides are deeply golden, about 2 to 4 minutes (if it’s taking forever to brown, raise the heat a bit; if it’s browning very quickly or you catch any whiff of smoke, turn down the heat). Carefully flip each piece of cheese with the tongs, recover the skillet, and cook until the other side is deeply golden, about 2 to 4 minutes.
Step 5 In the meantime, place a few paper towels on an empty plate or cutting board to absorb excess oil. Place each piece of cooked cheese onto the plate, sprinkle heavily with freshly ground black pepper, and let the cheese cool down a bit before handling.
Step 6 To assemble your sandwiches, first spread mayonnaise generously over two slices of bread (if making two sandwiches) or three (if making three sandwiches).
Step 7 Top each of the remaining plain slices of toast with two to three lettuce leaves. Top the lettuce with tomato slices, lightly overlapping them for full coverage. Top the tomatoes with slices of cheese, again overlapping them for full coverage. Top with the remaining slices of bread, mayo side down.
Step 8 Enjoy the sandwich whole, or use a sharp chef’s knife to slice them in half. These sandwiches are best served fresh, but will keep for about 24 hours in the refrigerator.
