Pat chicken (3 pounds) completely dry and sprinkle each side with salt and pepper.
Turn instant pot to saute, and add 2 tablespoons of oil. Once shimmering, add the chicken and sear each side until golden. You'll need to do this in batches so the IP doesn't get crowded. The chicken won't cook through, we're just looking for a good sear here. Don't be afraid to add more oil if the surface is looking dry. Place chicken on a plate and continue until you've seared all the chicken. Psst - you can make this dish with frozen chicken, just skip this step and increase the pressure cooking time to 10 minutes.
Add 2 remaining tablespoons of oil to the IP, along with the onion (1/2). Saute until the onion is golden and soft, this should take about 5 minutes. Use the back of your spoon to scrape up the brown bits from the chicken as the onion releases its liquid. Add the garlic (2) and toast until fragrant, about 1 minute.
Quickly add the diced tomatoes (1 14 ounce can), chipotle peppers in adobo (2), and chicken broth (1/2 cup). Press cancel on the IP. Nestle the chicken into the liquid. Seal, and select high pressure. Cook for 10 minutes. Once done, turn the pressure release valve to venting, and once the pin drops, remove the lid.
Place cooked chicken on a cutting board and shred with 2 forks when cool enough to handle. Tent loosely with foil as we finish our sauce.
Place saute ingredients in a blender and blend until smooth. Add the sauce back to the IP, along with the chicken, and toss to coat. Taste the sauce and adjust to your liking, I usually add 1/8th teaspoon of salt here.
To serve, spoon chicken into tortillas and top with your faves. Enjoy!