Poaching the chicken -- Put the chicken breasts in a heavy sauce pan, and add enough water to cover. Bring to a boil over high heat. Then reduce the heat, add the salt and pepper, and let simmer until the chicken achieves 165°F -- about 20 minutes. Reserve the poaching liquid. Meanwhile, shred the chicken and set it aside.
Making the sauce -- Tip the oil into a heavy pot or Dutch oven set over medium heat. Add the onion and then cover the pot, lower the heat, and let the onion sweat until soft but not colored -- about 8 minutes. Stir in the tomatoes, garlic, and 1/2 cup of the reserved chicken poaching liquid. Then stir in 2-3 chipotle peppers (use more or less depending on taste), and 2-3 teaspoons of the adobo sauce. Bring to a boil. Then lower the heat and let the sauce simmer for 10 minutes. Off heat, stir in the cilantro.
Transfer the sauce mixture to the jar of an electric blender. Blend at high speed until smooth -- about 30 seconds. Return the sauce to the pot or Dutch oven. Add the shredded chicken, and heat just until the chicken is warm throughout -- 2-3 minutes.
Assembling the tostadas: For each serving, spread a crisp tortilla with sour cream. Then spoon on some of the chicken-and-sauce mixture. Top with sliced raw onion, sliced cabbage, and crumbled queso fresco,