Place the drained are rinsed beans into a mixing bowl. Using the black of a fork, smash the beans until they are mushy, but still have lumps.
Add in the salt, black pepper, sour cream, mayonnaise,chopped chives, and chopped dill pickles. Toss all together until well combined.
Lightly smear each slice of bread with mayonnaise, if desired.
Evenly distribute the bean mixture over 4 slices of the bread. Top each of those sliced with sliced tomatoes, watercress, and microgreens. Place the remaining slices of bread on top and cut sandwiches in half .
Plate and serve with a dill pickle.