Step 1Combine the mayonnaise, lemon juice, garlic and 1/8 teaspoon of pepper. Lightly spread the garlic mayonnaise on one side of each slice of bread.
Step 2Cut the avocado in half. Remove the pit and peel. Cut each half in two lengthwise. Slice one quarter of the avocado onto each of 4 bread slices, using a fork to mash it.
Step 3Top the avocado with the fennel, carrots, red onion and cucumber slices, dividing the ingredients evenly among the sandwiches. Lightly sprinkle with salt and pepper. Top with the remaining bread slices. Cut the sandwiches in half and serve.