In a skillet heat oil over medium heat. Add tomatoes, ¼ of an onion, and garlic cloves. Fry until softened and slightly charred, about 5 minutes.
Add the cooked mixture to a blender with 1 tsp salt, pepper, chipotle peppers, oregano, ½ cup chicken stock, and chicken bouillon. Blend until smooth.
Shred the rotisserie chicken and add to a bowl. Set aside.
In a skillet heat oil over medium heat and add the remaining onion. Fry the onions until translucent.
Add the sauce and a pinch of salt, bring to simmer, and cook for a few minutes.
Add the shredded chicken and mix well. Adjust seasoning to taste and cook covered over medium-low heat for 5 minutes.
Add the tinga to tostadas with desired toppings and serve.
