Let's cook.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the thinly sliced white onion and cook for 3-5 minutes, until softened and translucent.
Add the minced garlic to the skillet and cook for 1 minute more, until fragrant.
Stir in the diced tomatoes (undrained), chopped chipotle in adobo sauce, dried oregano, and salt. Bring the mixture to a simmer.
Add the cooked shredded chicken to the skillet, stirring to coat it evenly with the sauce. Reduce heat to low and simmer for 5-7 minutes, allowing the flavors to meld and the sauce to thicken slightly.
While the chicken tinga simmers, arrange the tostada shells on serving plates.
Spoon the chicken tinga mixture evenly over each tostada shell.
Garnish with crumbled cotija cheese and chopped fresh cilantro before serving immediately.
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