Add the fire-roasted diced tomatoes, chipotle peppers (1–3 to taste), adobo sauce, chopped white onion, garlic cloves, ground cumin, and Mexican oregano to a high-speed blender. Blend on high until completely smooth, about 1 minute.
Heat the large skillet over medium-high heat and add the olive oil. Pour the blended sauce into the skillet and simmer, stirring occasionally, until it is lightly bubbling and has reduced and thickened slightly, about 5–7 minutes.
Add the cooked shredded chicken to the skillet and stir to coat in the sauce. Cook until the chicken is warmed through and evenly coated, about 3 minutes.
Warm or char the corn tortillas: hold each tortilla over a gas flame for 10–15 seconds per side or heat them in a dry skillet on an electric stove until warmed through.
Assemble tacos by dividing the sauced chicken among the tortillas and topping with diced red onion, minced cilantro, crumbled cotija, and a squeeze of lime. Serve immediately.
