Stir together the mayonnaise and harissa. Spread onto the bread.
Pile the grated cheese onto two of the slices of bread, and add the sliced tomatoes. Add the bacon for the omnivore's sandwich, and top with the remaining two bread slices.
Melt a little butter in a heated skillet over medium heat. Place the sandwiches in the skillet and loosely cover. Cook for five minutes. Flip, using a spatula, loosely cover, and cook for another five minutes.
Remove and slice in half.
I served mine with a large green salad with avocado slices, dressed with oil, vinegar and mustard.