Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion, poblano chile, and garlic to the pan; cook 6 minutes or until vegetables are tender, stirring frequently.
Stir in chile powder, salt, diced tomatoes, and tomato sauce. Using a potato masher, press diced tomatoes until well blended into the tomato sauce. Add 1 cup of chicken stock and bring to a simmer.
Add chicken, corn, and beans to the skillet and simmer for 20 minutes.
Place one tostada on a microwave-safe dish. Add 1/6th of the mixture to the shell and spread to thinly cover, and add 1/6th of the shredded cheese to that layer. Repeat two more times on the 1st stacker.
With the remaining tostada shells, build another stacker.
Place dish into microwave and heat on high for two to three minutes.
Cut each stacker into two.
Top with any of the optional toppings and serve.