Heat the oil in a large skillet over medium heat and add the onions. Cook until translucent and then lower the heat to medium low, and cook, stirring often, until the onions are nice and lightly caramelized. This will take about 15 minutes.
While the onions are cooking, add the tomatoes and chipotle peppers in adobo sauce to a food processor or blender. Puree until smooth.
Add the sauce to the onions and stir. Then add the chicken and cook for 10 minutes.
Serve on tostadas, or in tacos or gorditas, as desired.
