This sandwich is only going to taste as good as the ingredients you are using. Get a nice crusty whole grain bread, juicy flavorful heirloom tomatoes and a perfectly rip avocado to whip up an easy guacamole to use as a spread.
Cut the avocado in half and discard the pit. Scoop the flesh into a bowl and mash together with the scallion, cilantro, a pinch of sea salt and a good squeeze of lemon to taste. Refrigerate until needed.