Make the avocado dressing by combining the avocado, sour cream, chicken broth, lemon juice, 2 teaspoons of vegetable oil, sea salt, and freshly cracked pepper to taste in a bowl, then mix with an immersion blender (or food processor) until creamy and smooth. Set aside.
Make the crispy tostada shells by heating 1 teaspoon of vegetable oil in a pan over medium-high heat, and cook the tortilla for 3 minutes on each side or until golden brown. Repeat the procedure for the remaining tortillas. Set aside.
Prepare the chicken filling by adding the remaining 1 tablespoon of vegetable oil to the skillet and heating it over medium heat. Cook the onion for 4-5 minutes, stirring often.
Add the shredded chicken, black beans, cumin, and coriander to the chicken mixture; cook, stirring occasionally, for 2-3 minutes or until warmed through. Season with sea salt and freshly cracked pepper, to taste.
Prepare the tostada by layering the chicken mixture over a crispy tortilla, then add some lettuce, tomatoes, cilantro, avocado sauce, and cotija cheese crumbles. Serve immediately. Enjoy.