Step 1

Heat a cast-iron grill pan or skillet over medium-high heat. While the grill pan/skillet is pre-heating, continue with the next steps. (If you are not using cast-iron, see note 3.)

Step 2

Add the mayonnaise, lemon juice, garlic powder, a pinch of black pepper, and a few dashes hot sauce (optional) to a small bowl. Mix well to combine. Taste the mayo and add more lemon, pepper, and/or hot sauce if desired. Set the mayo aside.

Step 3

Pat the halloumi slices dry, then brush both sides of the halloumi and onions slices with oil and sprinkle both sides with black pepper.

Step 4

Oil the grill pan or skillet well. Place the halloumi slices, onion slices, and bread slices on the pre-heated grill pan or skillet. If you don't have enough room on your pan and need to cook the ingredients in stages, save the halloumi to cook last.

Step 5

Let the ingredients cook, undisturbed, for 2 - 3 minutes, or until nice grill marks appear.

Step 6

Flip the ingredients and cook for an additional 2 - 3 minutes. The halloumi should have a soft appearance, and when you touch it with tongs or a spatula, it should have some give to it. Remove the ingredients onto a plate.

Step 7

Spread the mayo onto one side of each slice of bread. Layer the halloumi, grilled onions, lettuce and tomato onto the sandwich and serve immediately.