Place ¼ piece of onion in a sauté pan with 1 ½-inch water over medium heat. Add chicken breast and simmer for 25 minutes, turning the chicken breast every few minutes. Remove from heat and let cool. When chicken breast has cooled, shred with two forks and set aside.

Finely chop remaining ¼ piece onion. In a separate pan, heat oil over medium heat. Add chopped onion; cook for 3 minutes. Add tomato, stir, cook for 3 minutes. Add watercress and stir until all ingredients have mixed well; about 3 minutes.  Add shredded chicken breast, oregano, garlic salt and pepper. Stir and continue cooking for 5 minutes. Remove from heat and set aside.

Spread 1/3 cup beans on a corn tostada. Top with chicken mixture, avocado slices and a sprinkle of cotija cheese. Repeat with remaining tostadas and chicken mixture. Decorate with watercress sprigs. Serve.