Heat olive oil in a large skillet over medium-high heat. Add chicken thighs and cook until browned on both sides. Remove chicken from skillet and set aside.
Add chopped onion to the skillet and cook until softened, about 5 minutes. Add minced garlic and cook for 1 minute more until fragrant.
Stir in diced tomatoes, minced chipotle peppers, adobo sauce, oregano, and cumin. Bring to a simmer.
Return the chicken thighs to the skillet. Reduce heat to low, cover, and simmer for 20-25 minutes, or until chicken is cooked through and tender.
Remove chicken from the skillet and shred it using two forks. Return the shredded chicken to the sauce and stir to combine. Season with salt and pepper to taste.
Warm the corn tortillas according to package directions.
Fill each tortilla with the chicken tinga mixture and add your favorite toppings. Serve immediately.
Notes
For a milder flavor, use only one chipotle pepper. For more heat, add an extra pepper or more adobo sauce.
You can cook the chicken ahead of time and shred it later.
This chicken tinga filling is also great in burritos or on top of rice bowls.