Put the chicken in a 6- to 8-quart slow cooker and sprinkle with 1 teaspoon cumin, 1/2 teaspoon salt and a few grinds of pepper. Add the onion, tomatoes, chipotle and adobo sauce, oregano, the remaining 1 teaspoon cumin and 1/2 teaspoon salt. Cover and cook on low until the chicken is tender and the sauce is slightly thickened, about 7 1/2 hours. Add the corn and stir until warmed through.
Warm the tortillas in a dry skillet or in the microwave. Top each serving of stew with cheese, avocado and cilantro. Serve with the warm tortillas.