Get your ingredients ready and start the oven on High Broil mode.
Spread a thin layer of French mustard on all 8 bread slices, then cover with mozzarella cheese and place in oven on high broil for 3 minutes. Depending on the proximity to the heat resistance the cheese and bread will melt faster. Monitor every 30 seconds until you get full melting and a light crust.
Cut the avocado in half, carve the edible area and place it into a bowl. With a fork, mush it, then add the lemon juice and diced onions. If the lemon is small you can use the full juice, if it is larger you may need it all. The guacamole may become too liquid.
Add the guacamole to the toasted bread and cheese on 4 of the 8 bread and cheese slices.
Put the remaining 4 slices on top so the guacamole fits in the center.