Warm tostadas according to package directions. Set aside.
Heat 1 tablespoon of olive oil in a small sauce pan over medium heat. Add half of the diced onion. Cook 3-5 minutes or until softened. Add in beans, chili powder, cumin, cayenne pepper and dried oregano. Cook until beans are warmed through. Remove from heat. Using a potato masher, mash beans. The mixture will be chunky. Season to taste with salt and pepper. Return to low heat and cover while you prepare the chicken.
Heat remaining tablespoon of olive oil in a skillet over medium heat. Add remaining onion and cook 3-5 minutes or until softened. Add chicken, cilantro and lime juice. Season to taste with salt and pepper. Cook until chicken is warmed through.
Preheat your oven broiler.
To assemble the tostada: spread thin layer of bean mixture on top of tostada shell top with cheddar cheese. Cook in the oven until cheese melts, about 3 minutes. Top with chicken mixture, lettuce, avocado and diced tomatoes.