For the chicken: Place the chicken, peppercorns, salt, garlic, bay leaf and onion in a large heavy saucepan with 6 cups of water. Bring to a boil over high heat, and then reduce to a simmer over medium-low heat for 20 minutes. Turn the heat off and remove the chicken from the liquid. Reserve the broth for later use. Using 2 forks or clean hands, shred the chicken into a bowl and set aside.
For the tinga: In a large heavy saute pan, cook the chorizo over medium heat until fully cooked, about 6 minutes. Add the onions and saute until translucent and fragrant, about 3 minutes. Add the garlic and saute for 1 minute. Add the tomatoes and cook for 1 minute. Add the tomatillos and cook for 1 minute. Add the chipotle, salt, pepper, Mexican oregano and the sprigs of marjoram and thyme. Add the shredded chicken. Continue cooking for another 3 minutes. Add between 1/2 cup and 1 cup of the chicken cooking liquid, depending on the thickness of the sauce. Cook for 5 more minutes. Remove the marjoram and thyme sprigs.
To assemble: Place 2 tablespoons of tinga on each tostada. Serve immediately.