Prepare the Spice Mix: In a small bowl, combine the smoked paprika, ground cumin, dried oregano, mild chilli powder, salt, and black pepper. Mix well until everything is evenly distributed.

Season the Chicken: Pat the chicken breasts dry with a paper towel and place them in a large bowl. Sprinkle over the spice mix and toss until the chicken is thoroughly coated on all sides.

Sauté the Aromatics: Heat the olive oil in a small frying pan over a medium heat. Add the finely chopped onion and cook for 5-7 minutes, until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Create the Sauce Base: Stir the tomato purée and chipotle paste into the onions and garlic, and cook for one minute. This step helps to cook out the raw taste of the purée. Pour in the chopped tomatoes and chicken stock. Bring to a gentle simmer and let it bubble for 2-3 minutes.

Combine in the Slow Cooker: Place the seasoned chicken breasts in the bottom of your slow cooker pot in a single layer. Pour the prepared tomato and onion sauce over the top, ensuring the chicken is mostly covered.

Cook Low and Slow: Put the lid on the slow cooker and cook on the LOW setting for 5-6 hours or on the HIGH setting for 3-4 hours. The chicken is done when it is cooked through and very tender. You can check the internal temperature to be sure; according to the Food Standards Agency, it should reach 75°C.

Shred the Chicken: Carefully remove the chicken breasts from the slow cooker and place them on a large plate or cutting board. I find that letting the chicken rest for five minutes makes it so much easier to shred. Using two forks, pull the meat apart until it's fully shredded.

Finish the Dish: Return the shredded chicken to the slow cooker and stir it into the sauce. Squeeze in the fresh lime juice and stir again to combine everything. Let it sit for another 10 minutes in the sauce on the 'keep warm' setting to absorb all that flavour. Serve hot with warm tortillas and your chosen toppings.