Press the [Sauté] button on your Instant Pot. Once hot, add the oil, onion, and garlic. Sauté for 3-4 minutes, occasionally stirring, until onion and garlic are fragrant. Add the chipotles, oregano, and cumin and stir. Add in the tomatoes, stock, and salt, and stir. Sautee/simmer for about 3 minutes. Press [Cancel] on the instant pot.
Transfer the tomato-onion-chipotle mixture to a high-powered blender, and blend until smooth.
Place raw chicken breasts in the instant pot and pour the blended sauce over them. Secure the lid. Set the pressure release valve to “sealing.” Select [Manual] or [high pressure] and cook on high pressure for 10 minutes. Use a quick release.
Once the steam has been completely removed, remove the lid. In the pot, shred the chicken using two forks (or however you like to shred chicken).
To assemble, top the tortillas with chicken and preferred toppings, such as chopped avocado, cilantro, and red onion. Serve with a lime wedge for squeezing.
If you have any chicken left over, as I did, you can use it to make burrito bowls, tacos, sandwiches, wraps, etc.