Heat the olive oil in a big pan over medium heat. Add the onion and cook until it's soft, about 5-7 minutes.
Add the garlic and cook for about 30 seconds more, until it smells good.
Pour in the tomatoes (with the juice!), chicken broth, chopped chipotle peppers, adobo sauce, oregano, cumin, salt, and pepper. Stir it all up.
Bring the sauce to a simmer, then turn the heat down to low. Let it cook for about 10-15 minutes, stirring sometimes. The sauce will get a little thicker.
Add the shredded chicken to the pan and stir it around so it's covered in sauce. Keep simmering for another 5-10 minutes.
While the chicken is simmering, get your toppings ready. Chop everything up and have it ready to go.
Put a tostada shell on a plate and put some chicken on top. Then, add all your favorite toppings.
Serve right away and enjoy!