Place the sliced onion and chicken strips in the bottom of the Crock-Pot and set it to LOW.
In a bowl, combine salsa, lime juice, minced jalapeño, minced garlic, taco seasoning, and salt. Pour this mixture over the chicken and onions, tossing to ensure everything is well coated. Cover with the lid and cook for 1 to 1 1/2 hours.
Once the chicken is mostly cooked, stir in the sliced bell peppers. Continue cooking for an additional hour.
Optional: For a thicker sauce, remove the lid after the cooking time and let it simmer for another 30 minutes. Alternatively, you can drain the liquid into a saucepan and reduce it over medium heat to your desired consistency.
Serve the filling on the tortillas with your choice of toppings. Guacamole, sour cream, sliced chilis, and crisp lettuce are all excellent ideas.