Cut the coriander leaves and set aside for later. Place the stalks and the remaining ingredients except the steak in a blender and pulse until it forms a coarse paste. Place the steak in a glass dish and cover both sides in the marinade. Leave to marinate for at least an hour.
Mix the vinegar, water and sugar together until the sugar has fully dissolved. Slice the red onions as thinly as possible and add to the pickling liquor. Allow to pickle as long as possible. They will eventually turn pink.
Remove the seeds from the tomatoes by cutting them in 1/4 and scooping them out. Chop the tomatoes in to a small dice. Peel and chop the red onions. Chop the coriander leave and mix with the tomato and red onion. Add the olive oil and the lime juice and season with the salt. Mix together and store in the fridge until needed.
Peel the avocados and crush with the back of a folk until and chunky paste. Peel and chop the red onion. Chop the coriander and add to the crush with the onion, lime juice and crème fraiche. Mix together and store in the fridge until needed.
Heat a heavy bottomed pan with a little oil. Remove the steak from the marinade, taking off any larger chunks. Sear the steak for 3 minutes on each side. Remove from the pan and allow to rest for 4 minutes. Meanwhile toast the tortillas on both sides in a hot, dry pan. A little charring will add flavour. Cut the lime in to wedges and grill in the same pan. Carve the steak thinly and place on the tray with the crushed avocado, tomato salsa, pickled red onions and toasted tortillas. Finish with the chard limes, a good spoon of crème fraiche and the remaining coriander leaves and then serve.