In a large skillet, on medium-high heat, add 1 tablespoon of oil and chicken. Cook for about 5 minutes and stir in taco seasoning. Remove chicken from the skillet.
Add another tablespoon of oil to the same skillet, and add in the onions, corn, and black beans. Sauté for about 5 minutes, or until onions are translucent, and set aside.
Evenly distribute the chicken and corn mixture on each tostada and top each one with cheese, diced tomatoes, chopped avocado, and sour cream.
Garnish with fresh cilantro, if desired, and serve.
Enjoy!