If your bread is not already toasted, toast the 4 slices. Lay the toasted slices on a work surface.
Prepare the produce: peel and grate enough carrot to equal 1/2 cup; peel and slice the cucumber into 8 slices; slice the avocado into 6 slices; slice the tomato into 4 slices; rinse the small handful of sliced red onion if using; measure out 1 cup spinach and 1/2 cup broccoli or alfalfa sprouts.
Spread the 1/3 cup hummus evenly over the inner side of each of the 4 bread slices.
Decide which two slices will be the sandwich bottoms and leave their hummus sides facing up. Divide the remaining ingredients evenly between the two bottoms as follows: 1/2 cup spinach per sandwich (total 1 cup), 1/4 cup grated carrots per sandwich (total 1/2 cup), 4 cucumber slices per sandwich (total 8), 3 avocado slices per sandwich (total 6), 2 tomato slices per sandwich (total 4), and 1/4 cup sprouts per sandwich (total 1/2 cup). If using cheese, place 1 slice of Havarti or provolone on each sandwich (total 2 slices). If using red onion, divide the rinsed slices between the two sandwiches.
Layer the ingredients on each bottom slice in this order (from bottom up): spinach, cheese (optional), grated carrots, cucumber slices, tomato slices. Sprinkle a light pinch of salt to taste over the tomato or cucumber layer.
Continue layering: add the avocado slices, then the red onion (optional), and finally the sprouts.
Place the remaining two bread slices on top of each assembled sandwich, hummus side down. Press gently, cut if desired, and serve immediately.