Butter your bread slices and toast them on a large pan on a medium heat. You can also do it in the air fryer at 400 F / 200 C for 3 to 4 mins. Do not over toast.
Meanwhile, fine chop onions, tomatoes, jalapenos, garlic and cilantro. Rinse your lettuce and shake off to drain excess water.
Cut open the avocado and remove the stone. Scoop the flesh to a mixing bowl.
Mash with a fork or a masher to a slightly coarse texture. Add all the other ingredients - onions, tomatoes, cilantro, jalapenos, garlic, salt and black pepper.
Mix well and taste test to adjust salt and lemon juice.
Place a lettuce over the toasted bread slice and scoop half of the guacamole. Spread it and cover with another slice of toasted bread.
Serve guacamole sandwich immediately. You can also keep this for about an hour an room temperature, if you toast your bread really well.