For the guacamole: Slice avocados in half and remove the pit. Scoop flesh into a bowl; mash with a fork. Mince onion and garlic. Add to avocado along with cilantro leaves, lime juice, salt and pepper; mix well. Halve tomato and discard the seeds; chop. Stir into guacamole.

Heat a sandwich press or griddle to medium heat. Spread an even amount of guacamole to each of 4 slices of bread. Add cheese and top with second slice of bread. Butter the outside of each sandwich. Place onto hot sandwich press and cook until golden brown and cheese is melted. Repeat with remaining sandwiches.

Serve grilled cheese sandwiches with clementines on the side.