Prepare the chicken. Season the chicken with salt and pepper. Coat both sides with flour, shaking off the excess.

Heat 1 tablespoon oil in a Dutch oven or braising pot over medium heat. Sear the chicken until browned on both sides. Remove the chicken and set aside.

In the same pot, add the sliced onion and garlic and saute until the onion is softened, about 5 minutes. Add back the chicken along with the beer, canned tomatoes, canned diced green peppers, brown sugar, chili powder, paprika, cumin, chipotle and adobo sauces, and oregano. Cover the pot and bring it to a boil. Reduce the heat to low and continue to simmer for 20 minutes or until the chicken is done.

Remove the chicken from the pot and increase the heat to medium-high. Continue to simmer the braising liquid for 10-15 minutes or until reduced by half.

Shred the chicken and throw it back into the reduced braising liquid. Season once more with salt and pepper and keep warm.

Meanwhile, prepare the toppings. Toss the shredded kale with juice of ½ a lime, olive oil, salt, and pepper. Pile the shredded chicken on the tostadas and top with shredded kale, pickled onions, sliced avocado, and cotija. Serve immediately.