Make the avocado dressing by combining the sour cream, chicken broth, lemon juice, 2 teaspoons of vegetable oil, sea salt, and freshly cracked pepper, to taste, and the avocado together in a bowl then mix with an immersion blender (or food processor) until creamy and smooth. Set aside.
Make the crispy tortillas tostadas by heating 1 teaspoon 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 heat 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 often, for 2-3 minutes or until the mixture is warmed through. Season with sea salt and freshly cracked pepper, to taste.
Prepare the tostada by layering the chicken mixture on top of a crispy tortilla then add some lettuce, tomatoes, cilantro, avocado sauce, and cotija cheese crumbles. Serve immediately. Enjoy.