First, peel the avocado (I wash the outside first), take the seed out and mash the avocado up with a fork.
Add to it the lemon juice, cumin and salt.
Butter one side of the 8 bread slices.
Lay 4 slices buttered side down.
Top each with a slice of the cheese.
Spread each slice with 1/4 of the avocado mixture.
Slice the tomato and top each sandwich with a couple slices.
Heat up your griddle or frying pan, and cook for 3 or 4 minutes, flipping once, or until desired golden "toastyness" is achieved, and the cheese is nicely melted.