Pour the tomatoes into a medium saucepan. Place over high heat. While it comes to a boil, stir in the chipotle chile, soy sauce, vinegar, dried oregano, garlic powder, and ¼ teaspoon salt. As soon as it comes to a boil, reduce the heat to medium-high. Using a fork, break up the chipotle chile into small bits.
Meanwhile, shred 3 cups of meat from the rotisserie chicken. Shred it finely - no big chunks of chicken, we want individual strands here-and discard the skin. Add the shredded chicken to the pot and cook, stirring frequently, until the chicken is warmed through and most of the liquid has evaporated, 3 to 4 minutes.
Spread a big spoonful of refried beans over each of the tostadas, then sprinkle with salt because they're typically a bit bland. Layer a big spoonful of the chicken tinga on top, then top however you please!
Slow Cooking Method
To make this dish in a slow cooker, toss 1 ½ pounds boneless, skinless chicken breasts or thighs and everything except the beans and tostadas into a slow cooker, cover, and cook on low for 6 hours.
