Optional Step - Sauté Onions: Heat olive oil in a large pan over medium-high heat. Add the sliced onion with a pinch of salt and sauté until browned. Transfer the onions to the slow cooker.
Trim large pieces of fat and gristle from the chicken thighs (a little fat left is great for flavor). Add chicken, diced tomatoes, bell peppers, garlic, and all the seasonings to the slow cooker. Stir well, pressing everything into an even layer. Ensure chicken thighs are smooth-side down and not bunched up.
Cover and cook on high for 2 1/2 to 4 hours or low for 3 to 6 hours. Check that the chicken is fully cooked (165°F) and tender—it should shred easily.
Remove the chicken and shred or thinly slice it. Scoop out about 1 cup of liquid from the slow cooker and set it aside. Return the shredded chicken to the slow cooker and stir gently. Add lime juice and mix well. Gradually stir in as much or as little of the reserved liquid as needed to adjust the flavor and consistency. Taste and adjust the seasonings, adding sugar if needed.
Use a slotted spoon or tongs to serve the chicken and veggies on charred tortillas (see note 1). Top with your favorites, like shredded cheese, sour cream, pickled red onions, cilantro-lime sauce, guacamole, or avocado. Enjoy!
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Notes
Note 1: To char tortillas, place them over a gas flame or in a dry skillet. Cook each side for 10–20 seconds until lightly charred and warmed. Use tongs to flip them safely.Storage: Store leftover chicken and veggies in an airtight container in the fridge for up to 4 days. Keep tortillas and toppings separate. Reheat chicken in the microwave or on the stove.