Place the chicken thighs in the bowl of the slow cooker. Add the chopped onion, crushed garlic, chipotle in adobo, dried oregano, ground cumin, smoked paprika, and canned diced tomatoes. Mix well to combine, ensuring the chicken is evenly coated and submerged in the liquid.
Cover the slow cooker with the lid and cook on low for 6 hours or high for 3 hours.
While the chicken cooks, place the thinly sliced red onion in a heatproof bowl. Pour boiling water over the onions and let them sit for 30 seconds to soften. Drain well.
In a small bowl, mix together the white vinegar, sugar, and salt. Transfer the softened onions to a jar and pour over the pickling liquid, pressing the onions down so they’re fully submerged. Let them cool to room temperature, then cover and refrigerate for at least 30 minutes or up to a few days.
Once the chicken is cooked, carefully remove it from the slow cooker and transfer it to a bowl. Use two forks to shred the chicken.
Pour the cooking liquid into a blender and blend until smooth. Return the sauce to the slow cooker, then add the shredded chicken back in, stirring to coat thoroughly. Season with salt to taste.
Heat the tortillas directly over a gas flame or wrap them in foil and warm in a low oven.
Fill each tortilla with the shredded chicken tinga. Top with pickled red onions, avocado slices, and a squeeze of lime juice.