Cover the bottom of the slow-cooker pot with the onions and garlic. Cut the pork into two equal-sized pieces and rub them all over with the spice blend. Place the pork on top of the onions and add 100ml cold water. Cover with a lid and cook on high power for 4 hours or on low power for 8 hours.
Preheat the oven to 200°C/fan 180°C/gas mark 6. 30 minutes before the end of the pork cooking time, bake each tortilla on a baking tray for 8 minutes until crisp and golden brown.
Combine the pineapple, red onion, chilli and lime juice. Add 2 tbsp coriander and stir through.
When the pork is done, remove it from the slow cooker and place it in a large bowl, leaving the cooking liquid and onions behind. Shred using two forks, removing any large pieces of fat. Add some of the onions from the slow cooker and a spoonful or two of the cooking liquid to taste.
To serve, arrange the lettuce leaves on the tortillas followed by the pork, some of the sour cream and then the pineapple salsa. Garnish with the remaining coriander.