Place the beef on a board and lightly score one side.
Bash the cinnamon, cumin, paprika and oregano in pestle and mortar, then rub into the cuts in the beef. Season well with sea salt and black pepper, drizzle over a little oil and brown the brisket well in a large pan over a high heat.
Place the bay leaves, whole peppers, plum tomatoes (breaking them up with the back of a spoon as you go), and the stock in a large pot and bring to the boil.
Meanwhile, deseed half the chillies, then finely slice them all. Peel and finely slice the onions, then pick and finely chop the coriander.
Add the chilli and onion to the pan with the brisket, cook for 5 minutes, then transfer the lot to the large pot, cover, and leave to simmer for 4 to 4½ hours, or until beautifully tender, stirring occasionally and adding splashes of water to loosen, if needed.
Gently pull the beef apart using 2 forks. Remove the bay, add a little vinegar to taste, scatter over the coriander and adjust the seasoning to taste.
For the guacamole, squeeze the avocado flesh into a bowl, discarding the skin and stones. Pick and roughly tear in the coriander leaves, peel and finely gate in the red onion, and squeeze in the lime juice.
Have a taste, then tweak with more salt, pepper or lime juice, if needed. Roughly chop then scrunch in the tomatoes, picking out any large bits of skin.
Deseed and finely grate in the chilli flesh and mix well. Serve with the chilli, tortillas and dollops of yoghurt. Delicious served with a fresh green salad on the side, too.