To make the guacamole : Cut avocados in half. Remove seed. Scoop out avacado from the peel, put in a large bowl. Using a fork, mash the avocado. Add the onion, garlic, jalapeño, cilantro, lime juice, salt and pepper. Stir until well combined. Add the chopped tomato and stir.
Heat a pan or griddle to medium-high heat. Spread desired amount of guacamole on both slices of bread then top with cheese. Butter outer slices of bread and grill on one side for about 2 minutes or until golden and crispy. Flip the sandwich and grill until golden brown. Make the other sandwich the same way and serve warm.