In small bowl, toss together onion slices with lime juice and salt, to taste; let sit 5 minutes, or as long as overnight.
Top each piece of toasted whole-grain bread with 1/4 cup guacamole.
Place equal parts pickled red onions, crumbled Cotija cheese and radish slices on top of each piece of toast. Garnish with cilantro, if desired, and serve.