To make the pickled red onions-In a medium bowl, combine red onion slices, vinegar, sugar, and salt. Stir until well combined. Let sit until red onions are pickled, about one hour, stirring occasionally.
To make the sandwich-Top one slice of the bread with the chipotle hummus. Squeeze a little lime juice on the avocado slices and sprinkle with salt. Place the avocado slices on top of the hummus. Add the microgreens, cilantro, radish slices, and queso fresco. Top with the other slice of bread. Cut the sandwich in half, if desired. Enjoy!