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To make the glaze, soak the ancho chillies in boiling water for 15 mins. While they’re soaking, heat a large frying pan (no oil is needed). Add the onion slices and garlic. Season and let blacken on a medium-low heat for about 15 mins. Turn everything so all sides blacken. When the garlic is blackened, peel it. Add it to a food processor or blender with the charred onion, ancho chillies, coriander stems, cider vinegar, sugar and a good measure of salt. Blend until smooth, then scrape into a bowl.
Slice the pork and pineapple into 4cm pieces. Thread them onto the soaked skewers and brush liberally with the glaze. You won’t use all of it, but save the rest as a sauce for serving.
In a small bowl, mix the chopped onion, lime juice and coriander leaves. Mix together.
Heat a grill to high. Grill the skewers for 2-5 mins each side until chargrilled. Let them rest for 5 mins under foil, then serve with the fresh onion salsa and remaining glaze.